📝 [#5066] Update frontend documentation

Added a (short) section on how to add custom styling of admin components.

docs/developers/frontend/index.rst

@@ -145,3 +145,38 @@ This script can handle current and future NPM packages published in the
 .. _FormatJS: https://formatjs.github.io/
 .. _react-intl: https://formatjs.github.io/docs/getting-started/installation
 .. _react-intl storybook addon: https://storybook.js.org/addons/storybook-react-intl
+
+Admin styling
+=============
+Custom styling of admin components can be realized by adding/updating .scss files,
+which are located in ``path/to/open-forms/src/openforms/scss``.
+
+Adding a custom style
+---------------------
+See below for an example on adding custom styling for a ``div`` component in the admin:
+
+.. code-block:: html
+
+    <div className="json-dump-variables json-dump-variables--horizontal">
+        ...
+    </div>
+
+1. Create a file called ``_json-dump-variables.scss`` in ``../scss/components/admin``
+
+2. Add styling, for example::
+
+        .json-dump-variables {
+          display: flex;
+
+          &--horizontal {
+            align-items: center;
+            gap: 0.5em;
+          }
+        }
+
+3. To ensure it gets picked up, add an import of the file name (without underscore) to the ``_index.scss``
+file of the parent directory (in this case ``../scss/components/admin/_index.scss``)::
+
+        ...
+        @import './json-dump-variables';
+        ...