fix: improve contrast for all links, button and label text

[cuteshaun]

This PR improves dark mode accessibility to meet the minimum contrast ratio of 4.5:1.

Issues

• Link text and underline colors in dark mode didn’t meet the 4.5:1 contrast ratio requirement.
• Hover state on links was barely noticeable due to minimal styling difference.
• Green and red labels didn’t provide enough contrast with white text.
• The data-theme="dark" attribute was not consistently applied, causing dark mode styles to fail.

Changes

• Updated dark mode link and underline colors for better contrast and visibility
• Added hover underline animation (+1px thickness and offset)
• Improved label background colors for accessibility
• Introduced _sass/custom/variables.scss with named tokens ($link-color-dark, etc.)
• Explicitly set data-theme attribute on based on a selected theme

Closes #535

Verified links contrast with the WCAG contrast checker tool

[cuteshaun]

@samcunliffe, thanks for merging main into feature branch!

[samcunliffe]

@samcunliffe, thanks for merging main into feature branch!

@cuteshaun, I'm sorry about the unfriendly contributor experience! Our CI was a bit misconfigured ❌❌.

[samcunliffe]

Screencast: first tab is these changes, second tab is the current deployed version.

Screen.Recording.2025-06-10.at.07.10.07.mov

Looks really nice! Also improves the traffic lights' colour contrast.

[samcunliffe]

Yeah, thanks again for the really nice and fast contribution!

Are you planning to attempt to get these changes in upstream? Let us know and we'll all thumbs-up your PR(s)!

[samcunliffe]

Nice. Contrast ratios confirmed.

Dark-mode links are 5.87:1.
Traffic lights are now 5:61:1 and 4:74:1.

[cuteshaun]

Are you planning to attempt to get these changes in upstream? Let us know and we'll all thumbs-up your PR(s)!

Haha, thanks, really appreciate that! When you say “upstream,” do you mean the original repository for just the docs theme?

[paddyroddy]

This is a really nice piece of work

[paddyroddy]

This is so clean

[samcunliffe]

Are you planning to attempt to get these changes in upstream? Let us know and we'll all thumbs-up your PR(s)!

Haha, thanks, really appreciate that! When you say “upstream,” do you mean the original repository for just the docs theme?

Yes, exactly. There are a few PRs open, but I didn't spot anything specifically improving the accessibility (there are issues but no solutions afai can see).