Dark theme for dropdowns in the Web IDE

Add dark theme support for the dropdowns in the Web IDE

Acceptance Criteria

Dark theme should cover the below dropdowns:

• Select branch/MR dropdown

• File row dropdown

• File/File template dropdown (for certain files like .gitignore)

• Review dropdown (to compare changes with the MR or last changes)

• Also consider hover and active states for all the dropdowns mentioned above.

• Additionally it should also consider styling for help tooltip: