A simple tip to complete a broken GTK themes (you know, the ones that make some apps look like an old Windows 95 software). Of course, you should first check that the last versions of the engines needed are installed and that your theme is correctly copied in the /usr/share/themes folder.
sudo apt-get install gtk2-engines-*
Once done, follow the steps.
Download your favorite GTK theme and just copy it inside a complete one, say the *buntu’s pre-installed and rather neutral Greybird. Rename your folder and copy it back to the themes folder. Go to the Settings screen, chose and apply the new theme.
cp -avr /usr/share/themes/Greybird/ cp -avr Litestyle-Brave_Dark/* Greybird/ mv Greybird/ LiteDark/ sudo cp -avr /usr/share/themes/