Fix todos hover style in dark mode

What does this MR do?

Our page specific CSS don't have a dark mode variant. Rather than compiling a page specific CSS file for each theme, we make use of CSS variables in order to define the correct colors. There is prior art with the IDE Themes and eventually we can move to only use these variables.

In case one variable is undefined, we will fallback on the light theme value for it. Thus we just need to define the variables in the dark theme, since the fallback is the light theme.

This approach will allow us to create other themes, e.g. for accessibility (color blindness, high contrast) in the future.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #241970 (closed)

Closes #241970 (closed)