diff --git a/changes/6817.bugfix b/changes/6817.bugfix
new file mode 100644
index 00000000000..281dabd7666
--- /dev/null
+++ b/changes/6817.bugfix
@@ -0,0 +1,6 @@
+Fix theme settings. Options which were used to specify a CSS file
+with a base theme are replaced. Use altenatives below in order
+to specify **asset** with a base theme for application:
+
+* `ckan.main_css` replaced by `ckan.theme`
+* `ckan.i18n.rtl_css` replaced by `ckan.i18n.rtl_theme`
diff --git a/ckan/controllers/admin.py b/ckan/controllers/admin.py
index bed19630cc0..8f79ac4906a 100644
--- a/ckan/controllers/admin.py
+++ b/ckan/controllers/admin.py
@@ -35,11 +35,11 @@ def __before__(self, action, **params):
 
     def _get_config_form_items(self):
         # Styles for use in the form.select() macro.
-        styles = [{'text': 'Default', 'value': '/base/css/main.css'},
-                  {'text': 'Red', 'value': '/base/css/red.css'},
-                  {'text': 'Green', 'value': '/base/css/green.css'},
-                  {'text': 'Maroon', 'value': '/base/css/maroon.css'},
-                  {'text': 'Fuchsia', 'value': '/base/css/fuchsia.css'}]
+        styles = [{'text': 'Default', 'value': 'css/main'},
+                  {'text': 'Red', 'value': 'css/red'},
+                  {'text': 'Green', 'value': 'css/green'},
+                  {'text': 'Maroon', 'value': 'css/maroon'},
+                  {'text': 'Fuchsia', 'value': 'css/fuchsia'}]
 
         homepages = [{'value': '1', 'text': 'Introductory area, search, featured group and featured organization'},
                      {'value': '2', 'text': 'Search, stats, introductory area, featured organization and featured group'},
@@ -47,7 +47,7 @@ def _get_config_form_items(self):
 
         items = [
             {'name': 'ckan.site_title', 'control': 'input', 'label': _('Site Title'), 'placeholder': ''},
-            {'name': 'ckan.main_css', 'control': 'select', 'options': styles, 'label': _('Style'), 'placeholder': ''},
+            {'name': 'ckan.theme', 'control': 'select', 'options': styles, 'label': _('Style'), 'placeholder': ''},
             {'name': 'ckan.site_description', 'control': 'input', 'label': _('Site Tag Line'), 'placeholder': ''},
             {'name': 'ckan.site_logo', 'control': 'image_upload', 'label': _('Site Tag Logo'), 'placeholder': '', 'upload_enabled':h.uploads_enabled(),
                 'field_url': 'ckan.site_logo', 'field_upload': 'logo_upload', 'field_clear': 'clear_logo_upload'},
diff --git a/ckan/lib/app_globals.py b/ckan/lib/app_globals.py
index 3fddba54e1e..43719ebc03d 100644
--- a/ckan/lib/app_globals.py
+++ b/ckan/lib/app_globals.py
@@ -72,12 +72,23 @@
 # A place to store the origional config options of we override them
 _CONFIG_CACHE = {}
 
-def set_main_css(css_file):
-    ''' Sets the main_css.  The css_file must be of the form file.css '''
-    assert css_file.endswith('.css')
-    new_css = css_file
-    # FIXME we should check the css file exists
-    app_globals.main_css = str(new_css)
+def set_theme(asset):
+    ''' Sets the theme.
+    The `asset` argument is a name of existing web-asset registered by CKAN
+    itself or by any enabled extension.
+    If asset is not registered, use default theme instead.
+    '''
+    from ckan.lib.webassets_tools import env
+
+    assert env
+    if asset not in env:
+        log.error(
+            "Asset '%s' does not exist. Fallback to '%s'",
+            asset, 'css/main'
+        )
+        asset = 'css/main'
+
+    app_globals.theme = asset
 
 
 def set_app_global(key, value):
@@ -168,8 +179,8 @@ def get_config_value(key, default=''):
         get_config_value(key)
 
     # custom styling
-    main_css = get_config_value('ckan.main_css', '/base/css/main.css')
-    set_main_css(main_css)
+    theme = get_config_value('ckan.theme') or 'css/main'
+    set_theme(theme)
 
     if app_globals.site_logo:
         app_globals.header_class = 'header-image'
diff --git a/ckan/lib/helpers.py b/ckan/lib/helpers.py
index 063ed4d4583..2c28e86f99b 100644
--- a/ckan/lib/helpers.py
+++ b/ckan/lib/helpers.py
@@ -687,8 +687,8 @@ def is_rtl_language():
 
 
 @core_helper
-def get_rtl_css():
-    return config.get('ckan.i18n.rtl_css', '/base/css/main-rtl.css')
+def get_rtl_theme():
+    return config.get('ckan.i18n.rtl_theme', 'css/main-rtl')
 
 
 class Message(object):
diff --git a/ckan/logic/schema.py b/ckan/logic/schema.py
index 9c1cf759a06..35bf21a9395 100644
--- a/ckan/logic/schema.py
+++ b/ckan/logic/schema.py
@@ -741,7 +741,7 @@ def default_update_configuration_schema(
         'ckan.site_about': [ignore_missing, unicode_safe],
         'ckan.site_intro_text': [ignore_missing, unicode_safe],
         'ckan.site_custom_css': [ignore_missing, unicode_safe],
-        'ckan.main_css': [ignore_missing, unicode_safe],
+        'ckan.theme': [ignore_missing, unicode_safe],
         'ckan.homepage_style': [ignore_missing, is_positive_integer],
         'logo_upload': [ignore_missing, unicode_safe],
         'clear_logo_upload': [ignore_missing, unicode_safe],
diff --git a/ckan/templates/admin/config.html b/ckan/templates/admin/config.html
index 2fb72909f24..48f929cc1e1 100644
--- a/ckan/templates/admin/config.html
+++ b/ckan/templates/admin/config.html
@@ -13,7 +13,7 @@
 
       {{ form.input('ckan.site_title', id='field-ckan-site-title', label=_('Site Title'), value=data['ckan.site_title'], error=error, classes=['control-medium']) }}
 
-      {{ form.select('ckan.main_css', id='field-ckan-main-css', label=_('Style'), options=styles, selected=data['ckan.main_css'], error=error) }}
+      {{ form.select('ckan.theme', id='field-ckan-main-css', label=_('Style'), options=styles, selected=data['ckan.theme'], error=error) }}
 
       {{ form.input('ckan.site_description', id='field-ckan-site-description', label=_('Site Tag Line'), value=data['ckan.site_description'], error=error, classes=['control-medium']) }}
 
diff --git a/ckan/templates/base.html b/ckan/templates/base.html
index e3c9bf54a6e..38ee3591d2f 100644
--- a/ckan/templates/base.html
+++ b/ckan/templates/base.html
@@ -67,9 +67,8 @@
     #}
     {%- block styles %}
       {# TODO: store just name of asset instead of path to it. #}
-      {% set main_css = h.get_rtl_css() if h.is_rtl_language() else g.main_css %}
-      {# strip '/base/' prefix and '.css' suffix #}
-      {% asset main_css[6:-4] %}
+      {% set theme = h.get_rtl_theme() if h.is_rtl_language() else g.theme %}
+      {% asset theme %}
     {% endblock %}
 
     {% block head_extras %}
diff --git a/ckan/templates/macros/autoform.html b/ckan/templates/macros/autoform.html
index 7ffc4c59f5d..c469b814343 100644
--- a/ckan/templates/macros/autoform.html
+++ b/ckan/templates/macros/autoform.html
@@ -12,7 +12,7 @@
 
   {% set form_info = [
       {'name': 'ckan.site_title', 'control': 'input', 'label': _('Site Title'), 'placeholder': ''},
-      {'name': 'ckan.main_css', 'control': 'select', 'options': styles, 'label': _('Style'), 'placeholder': ''},
+      {'name': 'ckan.theme', 'control': 'select', 'options': styles, 'label': _('Style'), 'placeholder': ''},
       {'name': 'ckan.site_description', 'control': 'input', 'label': _('Site Tag Line'), 'placeholder': ''},
       {'name': 'ckan.site_logo', 'control': 'input', 'label': _('Site Tag Logo'), 'placeholder': ''},
       {'name': 'ckan.site_about', 'control': 'markdown', 'label': _('About'), 'placeholder': _('About page text')},
diff --git a/ckan/tests/controllers/test_admin.py b/ckan/tests/controllers/test_admin.py
index c0d0c91a031..e7bf519f9ff 100644
--- a/ckan/tests/controllers/test_admin.py
+++ b/ckan/tests/controllers/test_admin.py
@@ -63,7 +63,7 @@ def test_main_css_list(self, app, sysadmin_env):
         for option in style_select_options:
             assert option.string in STYLE_NAMES
 
-    def test_main_css(self, app, sysadmin_env):
+    def test_main_theme(self, app, sysadmin_env):
         """Select a colour style"""
 
         # current style
@@ -72,7 +72,7 @@ def test_main_css(self, app, sysadmin_env):
 
         url = url_for(u"admin.config")
         # set new style css
-        form = {"ckan.main_css": "/base/css/red.css", "save": ""}
+        form = {"ckan.theme": "css/red", "save": ""}
         resp = app.post(url, data=form, environ_overrides=sysadmin_env)
 
         assert "red.css" in resp or "red.min.css" in resp
diff --git a/ckan/views/admin.py b/ckan/views/admin.py
index 1b58adbb925..88f11ea7374 100644
--- a/ckan/views/admin.py
+++ b/ckan/views/admin.py
@@ -29,19 +29,19 @@ def _get_sysadmins():
 def _get_config_options():
     styles = [{
         u'text': u'Default',
-        u'value': u'/base/css/main.css'
+        u'value': u'css/main'
     }, {
         u'text': u'Red',
-        u'value': u'/base/css/red.css'
+        u'value': u'css/red'
     }, {
         u'text': u'Green',
-        u'value': u'/base/css/green.css'
+        u'value': u'css/green'
     }, {
         u'text': u'Maroon',
-        u'value': u'/base/css/maroon.css'
+        u'value': u'css/maroon'
     }, {
         u'text': u'Fuchsia',
-        u'value': u'/base/css/fuchsia.css'
+        u'value': u'css/fuchsia'
     }]
 
     homepages = [{
@@ -62,7 +62,7 @@ def _get_config_options():
 
 def _get_config_items():
     return [
-        u'ckan.site_title', u'ckan.main_css', u'ckan.site_description',
+        u'ckan.site_title', u'ckan.theme', u'ckan.site_description',
         u'ckan.site_logo', u'ckan.site_about', u'ckan.site_intro_text',
         u'ckan.site_custom_css', u'ckan.homepage_style'
     ]
diff --git a/doc/contributing/frontend/templating.rst b/doc/contributing/frontend/templating.rst
index c3df14cb87d..41cfe67e329 100644
--- a/doc/contributing/frontend/templating.rst
+++ b/doc/contributing/frontend/templating.rst
@@ -448,7 +448,7 @@ values for the (key, value, delete) fields respectively.
     classes     - An array of classes to apply to the control-group.
     attrs       - Dictionary of extra tag attributes
     is_required - Boolean of whether this input is required for the form to validate
-    
+
 
 Examples:
 
@@ -481,7 +481,7 @@ Example
 
     {% set form_info = [
         {'name': 'ckan.site_title', 'control': 'input', 'label': _('Site Title'), 'placeholder': ''},
-        {'name': 'ckan.main_css', 'control': 'select', 'options': styles, 'label': _('Style'), 'placeholder': ''},
+        {'name': 'ckan.theme', 'control': 'select', 'options': styles, 'label': _('Style'), 'placeholder': ''},
         {'name': 'ckan.site_description', 'control': 'input', 'label': _('Site Tag Line'), 'placeholder': ''},
         {'name': 'ckan.site_logo', 'control': 'input', 'label': _('Site Tag Logo'), 'placeholder': ''},
         {'name': 'ckan.site_about', 'control': 'markdown', 'label': _('About'), 'placeholder': _('About page text')},
diff --git a/doc/extensions/remote-config-update.rst b/doc/extensions/remote-config-update.rst
index e3fb66ca29c..fcce27f0cd2 100644
--- a/doc/extensions/remote-config-update.rst
+++ b/doc/extensions/remote-config-update.rst
@@ -34,7 +34,7 @@ First of all, let's call the :py:func:`~ckan.logic.action.get.config_option_list
         "help": "http://localhost:5000/api/3/action/help_show?name=config_option_list",
         "result": [
             "ckan.site_custom_css",
-            "ckan.main_css",
+            "ckan.theme",
             "ckan.site_title",
             "ckan.site_about",
             "ckan.site_url",
@@ -71,7 +71,7 @@ Restart the web server and do another request to the :py:func:`~ckan.logic.actio
             "ckan.datasets_per_page",
             "ckanext.example_iconfigurer.test_conf",
             "ckan.site_custom_css",
-            "ckan.main_css",
+            "ckan.theme",
             "ckan.site_title",
             "ckan.site_about",
             "ckan.site_url",