Sidebar headers are not visible enough

[nickvergessen]

Have a "dark" theming color and use dark mode

Expected

The sidebar is clearly visible

Actual

[nickvergessen]

Comments in the chat:

• @nimishavijay

  You are right, that color combination does not have enough contrast, the fallback colors should be triggered for such a dark color in dark mode. cc @marcoambrosini @szaimen

• @susnux

  I think we probably need to go the same way like with the button component which enforces primary style for checked state