Labels sometimes hard to read especially on dark themes

Steps to reproduce:

1.

• open QC with a bigger project - or at least a project with some bigger files
• have CCM enabled (seems to be involved)
• open a bigger file (e.g. in QC for me texteditor.cpp, editormanager.cpp, debuggerplugin.cpp will do..)
• check the Line/Column label on top of the editor widget - text is displayed black

2. (reproducible on master)

• open / create a small project
• debug it
• when interrupted / stopped at a breakpoint check the debugger toolbar
  • the 'Thread:' label and the QComboBox next to it are having black foreground colors

3. (reproducible on master)

• open / create a small project
• use a release / profile build and run callgrind on it
• after finishing there's a combobox added to the callgrind toolbar (before the cost format) that has the same problem (black foreground color on a dark theme)