Update usage page colors in dark theme

[gtsiolis]

Problem to solve.

Following up from #11481, the new usage page contains a table and a new sidebar design that does not render well in dark theme regarding color accessibility and information hierarchy.

Light Theme	Dark Theme	Dark Theme (Table Hover)

FWIW, the table used here could be using the same exactly table component as in the new blocked repositories layout (see #11446) which is also suffering from similar issues regarding the dark theme, see point [16] in #11463:

16. Improve dark theme colors. 🌔

[gtsiolis]

Cross-posting designs from #11640 (comment):

Dark theme

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[gtsiolis]

Closing this as usage layout and colors have been updated.