fix: docs features darkmode color

[fabianhug]

Relates to:

No Relation

Risks

None

Background

What does this PR do?

It fixes the fucked up font in the Docs / GitHub pages when using Darkmode.
No rocket science.

Styling improvements:

• Changed the card background color to use the CSS variable --ifm-card-background-color instead of a fixed RGBA value.
• Added a CSS variable --ifm-font-color-base for the card text color.
• Updated the heading color to use the CSS variable --ifm-heading-color.

Testing

Detailed testing steps

Check out locally, cd into docs, run dev and looky looky

Screenshots

Before

After