GUI: fix dialog contrast for dark color schemes

[evilmav]

CodeXL seems to override background colors e.g. for the "Create a new project" dialog, while keeping system-color fonts. This leads to unreadable dialogs for dark themes like KDE's breeze dark on linux:

[Boris-Barboris]

For people looking for a dirty workaround:

The simplest workaround under Plasma is just to use custom home:
mkdir ~/.codexlhome
XDG_CONFIG_HOME=$HOME/.codexlhome ./CodeXL