Fix color coding in dark and light theme

[steffen-wilke]

Because use #37474F as our highlight color in of the Material Theme, some places in the application now have either a very low color contrast or some controls do even have a confusing color coding where the colors actually suggest the opposite of what the control does:

Example 1: White is disabled

Example 2: Low color contrast causes almost invisible buttons

We need to investigate the entire application and make sure that the theme works to the purpose of a clean user experience.