DOC: Fix link to custom CSS example (#2025)

As pointed out in
#2023 (comment)

Fix #2023

docs/user_guide/styling.rst

@@ -35,6 +35,8 @@ To add a custom stylesheet, follow these steps:
 
 When you build your documentation, this stylesheet should now be activated.
 
+.. _css-variables:
+
 CSS theme variables
 ===================
 

docs/user_guide/web-components.rst

@@ -139,7 +139,7 @@ Here are some of the available button-style links, also using semantic colors:
    native button components such as using the correct `ARIA attributes
    <https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles/button_role>`__.
 
-If in your site's `custom CSS file <custom-css>`_ you override the `CSS custom properties <css-variables>`_ ``--pst-color-*`` (where ``*`` is one of the semantic color names, such as ``primary``, ``danger``), badges and buttons will automatically use the custom color.
+If in your site's :ref:`custom CSS file <custom-css>` you override the :ref:`CSS custom properties <css-variables>` ``--pst-color-*`` (where ``*`` is one of the semantic color names, such as ``primary``, ``danger``), badges and buttons will automatically use the custom color.
 
 Cards
 =====