Fix light mode TopHeader using --mantine-color-anchor instead of parent's color

As title. Changing navigation items from Container to Anchor in 41627e7d makes Mantine apply some link-specific styles. Specifically one that concerns us is color: var(--mantine-color-anchor), which overrides the color inherited from our parent. This is problematic since we depend on the previous behaviour to get white text in the TopHeader in light mode.

PR fixed it by applying color: inherit.