diff --git a/end-to-end-tests/tests/runtime/sidebarPanelTheme.spec.ts b/end-to-end-tests/tests/runtime/sidebarPanelTheme.spec.ts
new file mode 100644
index 0000000000..74360a015b
--- /dev/null
+++ b/end-to-end-tests/tests/runtime/sidebarPanelTheme.spec.ts
@@ -0,0 +1,54 @@
+/*
+ * Copyright (C) 2024 PixieBrix, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+import { test, expect } from "../../fixtures/extensionBase";
+import { ActivateModPage } from "../../pageObjects/extensionConsole/modsPage";
+import { getSidebarPage, runModViaQuickBar } from "../../utils";
+import type { Page } from "@playwright/test";
+
+test("custom sidebar theme css file is applied to all levels of sidebar document", async ({
+  page,
+  extensionId,
+}) => {
+  const modId = "@pixies/testing/panel-theme";
+
+  const modActivationPage = new ActivateModPage(page, extensionId, modId);
+  await modActivationPage.goto();
+
+  await modActivationPage.clickActivateAndWaitForModsPageRedirect();
+
+  await page.goto("/");
+
+  // Ensure the page is focused by clicking on an element before running the keyboard shortcut, see runModViaQuickbar
+  await page.getByText("Index of  /").click();
+  await runModViaQuickBar(page, "Show Sidebar");
+
+  const sidebarPage = (await getSidebarPage(page, extensionId)) as Page;
+  await expect(
+    sidebarPage.getByText("#8347: Theme Inheritance", { exact: true }),
+  ).toBeVisible();
+
+  const green = "rgb(0, 128, 0)";
+  const elementsThatShouldBeGreen = await sidebarPage
+    .getByText("This should be green")
+    .all();
+  await Promise.all(
+    elementsThatShouldBeGreen.map(async (element) =>
+      expect(element).toHaveCSS("color", green),
+    ),
+  );
+});
diff --git a/src/bricks/renderers/CustomFormComponent.tsx b/src/bricks/renderers/CustomFormComponent.tsx
index 9d081cb6eb..66263fa042 100644
--- a/src/bricks/renderers/CustomFormComponent.tsx
+++ b/src/bricks/renderers/CustomFormComponent.tsx
@@ -34,6 +34,7 @@ import DescriptionField from "@/components/formBuilder/DescriptionField";
 import TextAreaWidget from "@/components/formBuilder/TextAreaWidget";
 import RjsfSubmitContext from "@/components/formBuilder/RjsfSubmitContext";
 import { cloneDeep } from "lodash";
+import { useStylesheetsContextWithFormDefault } from "@/components/StylesheetsContext";
 
 const FIELDS = {
   DescriptionField,
@@ -65,6 +66,7 @@ export type CustomFormComponentProps = {
   resetOnSubmit?: boolean;
   className?: string;
   stylesheets?: string[];
+  disableParentStyles?: boolean;
 };
 
 const CustomFormComponent: React.FunctionComponent<
@@ -78,7 +80,8 @@ const CustomFormComponent: React.FunctionComponent<
   className,
   onSubmit,
   resetOnSubmit = false,
-  stylesheets,
+  disableParentStyles = false,
+  stylesheets: newStylesheets,
 }) => {
   // Use useRef instead of useState because we don't need/want a re-render when count changes
   // This ref is used to track the onSubmit run number for runtime tracing
@@ -99,6 +102,11 @@ const CustomFormComponent: React.FunctionComponent<
     setKey((prev) => prev + 1);
   };
 
+  const { stylesheets } = useStylesheetsContextWithFormDefault({
+    newStylesheets,
+    disableParentStyles,
+  });
+
   const submitData = async (data: UnknownObject): Promise<void> => {
     submissionCountRef.current += 1;
     await onSubmit(data, {
diff --git a/src/bricks/renderers/documentView/DocumentView.tsx b/src/bricks/renderers/documentView/DocumentView.tsx
index d1f65c5d24..fdb7cc3cba 100644
--- a/src/bricks/renderers/documentView/DocumentView.tsx
+++ b/src/bricks/renderers/documentView/DocumentView.tsx
@@ -23,10 +23,14 @@ import { type DocumentViewProps } from "./DocumentViewProps";
 import DocumentContext from "@/components/documentBuilder/render/DocumentContext";
 import { Stylesheets } from "@/components/Stylesheets";
 import { joinPathParts } from "@/utils/formUtils";
+import StylesheetsContext, {
+  useStylesheetsContextWithDocumentDefault,
+} from "@/components/StylesheetsContext";
 
 const DocumentView: React.FC<DocumentViewProps> = ({
   body,
-  stylesheets,
+  stylesheets: newStylesheets,
+  disableParentStyles,
   options,
   meta,
   onAction,
@@ -41,26 +45,33 @@ const DocumentView: React.FC<DocumentViewProps> = ({
     throw new Error("meta.extensionId is required for DocumentView");
   }
 
+  const { stylesheets } = useStylesheetsContextWithDocumentDefault({
+    newStylesheets,
+    disableParentStyles,
+  });
+
   return (
     // Wrap in a React context provider that passes BrickOptions down to any embedded bricks
     <DocumentContext.Provider value={{ options, onAction }}>
-      <Stylesheets href={stylesheets}>
-        {body.map((documentElement, index) => {
-          const documentBranch = buildDocumentBranch(documentElement, {
-            staticId: joinPathParts("body", "children"),
-            // Root of the document, so no branches taken yet
-            branches: [],
-          });
+      <StylesheetsContext.Provider value={{ stylesheets }}>
+        <Stylesheets href={stylesheets}>
+          {body.map((documentElement, index) => {
+            const documentBranch = buildDocumentBranch(documentElement, {
+              staticId: joinPathParts("body", "children"),
+              // Root of the document, so no branches taken yet
+              branches: [],
+            });
 
-          if (documentBranch == null) {
-            return null;
-          }
+            if (documentBranch == null) {
+              return null;
+            }
 
-          const { Component, props } = documentBranch;
-          // eslint-disable-next-line react/no-array-index-key -- They have no other unique identifier
-          return <Component key={index} {...props} />;
-        })}
-      </Stylesheets>
+            const { Component, props } = documentBranch;
+            // eslint-disable-next-line react/no-array-index-key -- They have no other unique identifier
+            return <Component key={index} {...props} />;
+          })}
+        </Stylesheets>
+      </StylesheetsContext.Provider>
     </DocumentContext.Provider>
   );
 };
diff --git a/src/bricks/renderers/documentView/DocumentViewProps.tsx b/src/bricks/renderers/documentView/DocumentViewProps.tsx
index 65fca3255c..f677846a6f 100644
--- a/src/bricks/renderers/documentView/DocumentViewProps.tsx
+++ b/src/bricks/renderers/documentView/DocumentViewProps.tsx
@@ -33,6 +33,10 @@ export type DocumentViewProps = {
    * Remote stylesheets (URLs) to include in the document.
    */
   stylesheets?: string[];
+  /**
+   * Whether to disable the base (bootstrap) styles, plus any inherited styles, on the document (and children).
+   */
+  disableParentStyles?: boolean;
 
   options: BrickOptions<BrickArgsContext>;
   meta: {
diff --git a/src/bricks/transformers/ephemeralForm/EphemeralFormContent.tsx b/src/bricks/transformers/ephemeralForm/EphemeralFormContent.tsx
index 689ba4c3e4..d4e2a42748 100644
--- a/src/bricks/transformers/ephemeralForm/EphemeralFormContent.tsx
+++ b/src/bricks/transformers/ephemeralForm/EphemeralFormContent.tsx
@@ -32,6 +32,7 @@ import DescriptionField from "@/components/formBuilder/DescriptionField";
 import RjsfSelectWidget from "@/components/formBuilder/RjsfSelectWidget";
 import TextAreaWidget from "@/components/formBuilder/TextAreaWidget";
 import { Stylesheets } from "@/components/Stylesheets";
+import { useStylesheetsContextWithFormDefault } from "@/components/StylesheetsContext";
 
 export const fields = {
   DescriptionField,
@@ -55,8 +56,21 @@ const EphemeralFormContent: React.FC<EphemeralFormContentProps> = ({
   nonce,
   isModal,
 }) => {
-  const { schema, uiSchema, cancelable, submitCaption, stylesheets } =
-    definition;
+  const {
+    schema,
+    uiSchema,
+    cancelable,
+    submitCaption,
+    stylesheets: newStylesheets,
+    disableParentStyles,
+  } = definition;
+
+  // Ephemeral form can never be nested, but we use this to pull in
+  // the (boostrap) base themes
+  const { stylesheets } = useStylesheetsContextWithFormDefault({
+    newStylesheets,
+    disableParentStyles: disableParentStyles ?? false,
+  });
 
   return (
     <Stylesheets href={stylesheets}>
diff --git a/src/components/StylesheetsContext.ts b/src/components/StylesheetsContext.ts
new file mode 100644
index 0000000000..d5a7c831da
--- /dev/null
+++ b/src/components/StylesheetsContext.ts
@@ -0,0 +1,109 @@
+/*
+ * Copyright (C) 2024 PixieBrix, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+import React, { useContext } from "react";
+import bootstrap from "@/vendors/bootstrapWithoutRem.css?loadAsUrl";
+import bootstrapOverrides from "@/sidebar/sidebarBootstrapOverrides.scss?loadAsUrl";
+import custom from "@/bricks/renderers/customForm.css?loadAsUrl";
+
+export type StylesheetsContextType = {
+  stylesheets: string[] | null;
+};
+
+const StylesheetsContext = React.createContext<StylesheetsContextType>({
+  stylesheets: null,
+});
+
+function useStylesheetsContextWithDefaultValues({
+  newStylesheets,
+  defaultStylesheets,
+  disableParentStyles,
+}: {
+  newStylesheets: string[] | undefined;
+  defaultStylesheets: string[];
+  disableParentStyles: boolean;
+}): {
+  stylesheets: string[];
+} {
+  const { stylesheets: inheritedStylesheets } = useContext(StylesheetsContext);
+
+  const stylesheets: string[] = [];
+
+  if (!disableParentStyles) {
+    if (inheritedStylesheets == null) {
+      stylesheets.push(...defaultStylesheets);
+    } else {
+      stylesheets.push(...inheritedStylesheets);
+    }
+  }
+
+  if (newStylesheets != null) {
+    stylesheets.push(...newStylesheets);
+  }
+
+  return { stylesheets };
+}
+
+export function useStylesheetsContextWithDocumentDefault({
+  newStylesheets,
+  disableParentStyles,
+}: {
+  newStylesheets: string[] | undefined;
+  disableParentStyles: boolean;
+}): {
+  stylesheets: string[];
+} {
+  return useStylesheetsContextWithDefaultValues({
+    newStylesheets,
+    defaultStylesheets: [
+      bootstrap,
+      bootstrapOverrides,
+      // DocumentView.css is an artifact produced by webpack, see the DocumentView entrypoint included in
+      // `webpack.config.mjs`. We build styles needed to render documents separately from the rest of the sidebar
+      // in order to isolate the rendered document from the custom Bootstrap theme included in the Sidebar app
+      "/DocumentView.css",
+      // Required because it can be nested in the DocumentView.
+      "/CustomFormComponent.css",
+    ],
+    disableParentStyles,
+  });
+}
+
+export function useStylesheetsContextWithFormDefault({
+  newStylesheets,
+  disableParentStyles,
+}: {
+  newStylesheets: string[] | undefined;
+  disableParentStyles: boolean;
+}): {
+  stylesheets: string[];
+} {
+  return useStylesheetsContextWithDefaultValues({
+    newStylesheets,
+    defaultStylesheets: [
+      bootstrap,
+      bootstrapOverrides,
+      // CustomFormComponent.css and EphemeralFormContent.css are artifacts produced by webpack, see the entrypoints.
+      "/EphemeralFormContent.css",
+      "/CustomFormComponent.css",
+      custom,
+    ],
+    disableParentStyles,
+  });
+}
+
+export default StylesheetsContext;
diff --git a/src/tsconfig.strictNullChecks.json b/src/tsconfig.strictNullChecks.json
index 182122cf49..2422965741 100644
--- a/src/tsconfig.strictNullChecks.json
+++ b/src/tsconfig.strictNullChecks.json
@@ -250,6 +250,7 @@
     "./components/StopPropagation.tsx",
     "./components/Stylesheets.test.tsx",
     "./components/Stylesheets.tsx",
+    "./components/StylesheetsContext.ts",
     "./components/TooltipIconButton.tsx",
     "./components/UnstyledButton.tsx",
     "./components/addBlockModal/TagList.tsx",