Fix homepage SegmentedControl after dark mode feature

The SegmentedControl on the homepage to select the sort mode has two problems after the dark mode feature was merged:

• The spacing beneath it has disappeared, making it very cramped against the category list
• The background colour of the root element in dark mode is the same as the page color (dark-8) so it's not distinguishable what is text vs a clickable area

This MR addresses both of these concerns.

The spacing was due to two removed lines in secondary-container.tsx which was removed (accidentally?).

The background colour is fixed through using Mantine's provided --sc-color and a new custom CSS variable that's resolved based on color scheme.

dark	light