Adjust header colors on light theme

- Use adjusted background-color for header instead of text.hint.
- Use slightly darker secondary font color for better readability against the
  head background color.

Use more semantic theme colors for bug list
Use more semantic theme colors for bug messages
Fix usage of text hint for filter header

This should prevent unnecessary merge conflicts.

- Use theme colors for title input.
- Remove inputTitle classes as they are not applied to the TextField.
  This will lead to double borders and artifacts at the field corners.

This will ensure better color managment for Message titles