docs: new styling docs / themes

[sissbruecker]

Adds the Themes article for the new styling docs.

Note

The PR targets a base branch for the new styling docs. That branch has the old styling docs removed and new articles are added gradually until the new section is ready. Cross-references between new articles will be filled in later, respective sections have been marked with TODOs.

[github-actions]

AI Language Review

The following issues require improvement in the Themes and Base Styles file:

1. Under "Aura Theme":

   • The closing bracket of the image caption for "Login form using the Aura theme" is incorrect. It should be image::_images/aura-theme.png[Login form using the Aura theme, width=50%] instead of image::_images/aura-theme.png[[Login form using the Aura theme, width=50%].

2. Under "Lumo Theme":

   • The descriptions and instructions for the Aura and Lumo themes are largely repetitive. Consider restructuring to avoid duplication by summarizing common instructions and only highlighting differences between the themes.

3. General:

   • Consistent usage of CSS comments is important. Currently, the CSS code snippets show inconsistent commenting for custom styles. It is recommended to use a uniform commenting style across all CSS blocks, providing context where necessary.

[sissbruecker]

Applying light + dark theme using Lumo needs more work.

Edit: Updated the Lumo section to recommend adding theme="dark" to the generated index.html.