Issue1294: Fixes to colors in Sphinx theme

[MatthiasZepper]

In order to address #1294, I have slightly modified the secondary colors of the light theme and added a few additional CSS selectors to ensure that there are no elements (like the hyperlinks) without associated styles. Those are styled with the default colors from the user's desktop theme and thus might not play well with the theme.

Secondary color change in light theme:

Orange to dark red: #f4a25b to #C03221
Lighter blue to darker blue: #6fb2e8 to #246EB9

Current light secondary color scheme:

Proposed light secondary color scheme:

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[ewels]

Brilliant, thank you! 👍🏻