Dark theme - Reduce colors in form based editor

[vogella]

IMHO we should use less colors in the new dark theme #2114 in form based editors. For example in the MANIFEST.MF editor.

@BeckerWdf @mvm-sap @thomasritter WDYT?

[vogella]

Areas for improvement:

Dark spacing between elements (marked in the screenshot):

Form header background

[mvm-sap]

This is a Linux-only issue, but it looks fine in Windows. Will check and fix it.

[vogella]

Thanks @mvm-sap