Use system theme colours for all GUI elements

[beefsack]

As attached, using Gnome (under Ubuntu) with Blackbird theme (dark theme).

It's correctly using light coloured text for a lot of the text on screen, but there are some issues with colouring on some text and element backgrounds, which seem hardcoded and not referencing system theme. From what I can see:

• Tree labels in left panel has non-system background
• Left panel under tree has non-system background
• Top right panel has non-system background, and label text has non-system colour (is black, would be unreadable if the background was dark)
• Results panel bottom left has non-system background

Thanks for the great program by the way :+1:

[agirbal]

thanks for report, sorry for delay getting to it. We may be hard coding the bg colors due to old bug in osx.