Improve headline hierarchy

Add headline specific Sass variables:

```scss
$h-size-1: 1.563em !default; // ~25.008px
$h-size-2: 1.25em !default; // ~20px
$h-size-3: 1.125em !default; // ~18px
$h-size-4: 1.0625em !default; // ~17px
$h-size-5: 1.03125em !default; // ~16.5px
$h-size-6: 1em !default; // ~16px
```

Fixes mmistakes#2423

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### Enhancements
 
+- Improve headline hierarchy and add Sass specific variables `$h-size-x`. [#2423](https://github.com/mmistakes/minimal-mistakes/issues/2423)
 - Improve accessibility of `default` skin by increasing color contrast of text and links.
 - Hide posts with `hidden: true` YAML front matter from appearing in listings. [#2345](https://github.com/mmistakes/minimal-mistakes/pull/2345)
 - Add Irish (Gaeilge) localized UI text strings. [#2422](https://github.com/mmistakes/minimal-mistakes/pull/2422)

_sass/minimal-mistakes/_base.scss

@@ -35,27 +35,27 @@ h6 {
 
 h1 {
   margin-top: 0;
-  font-size: $type-size-3;
+  font-size: $h-size-1;
 }
 
 h2 {
-  font-size: $type-size-4;
+  font-size: $h-size-2;
 }
 
 h3 {
-  font-size: $type-size-5;
+  font-size: $h-size-3;
 }
 
 h4 {
-  font-size: $type-size-6;
+  font-size: $h-size-4;
 }
 
 h5 {
-  font-size: $type-size-6;
+  font-size: $h-size-5;
 }
 
 h6 {
-  font-size: $type-size-6;
+  font-size: $h-size-6;
 }
 
 small,

_sass/minimal-mistakes/_variables.scss

@@ -43,6 +43,14 @@ $type-size-6: 0.75em !default; // ~12px
 $type-size-7: 0.6875em !default; // ~11px
 $type-size-8: 0.625em !default; // ~10px
 
+/* headline scale */
+$h-size-1: 1.563em !default; // ~25.008px
+$h-size-2: 1.25em !default; // ~20px
+$h-size-3: 1.125em !default; // ~18px
+$h-size-4: 1.0625em !default; // ~17px
+$h-size-5: 1.03125em !default; // ~16.5px
+$h-size-6: 1em !default; // ~16px
+
 /*
    Colors
    ========================================================================== */

docs/_docs/18-history.md

@@ -5,14 +5,15 @@ permalink: /docs/history/
 excerpt: "Change log of enhancements and bug fixes made to the theme."
 sidebar:
   nav: docs
-last_modified_at: 2020-03-04T16:35:36-05:00
+last_modified_at: 2020-03-05T12:39:27-05:00
 toc: false
 ---
 
 ## Unreleased
 
 ### Enhancements
 
+- Improve headline hierarchy and add Sass specific variables `$h-size-x`. [#2423](https://github.com/mmistakes/minimal-mistakes/issues/2423)
 - Improve accessibility of `default` skin by increasing color contrast of text and links.
 - Hide posts with `hidden: true` YAML front matter from appearing in listings. [#2345](https://github.com/mmistakes/minimal-mistakes/pull/2345)
 - Add Irish (Gaeilge) localized UI text strings. [#2422](https://github.com/mmistakes/minimal-mistakes/pull/2422)