Changing the default theme on Sparky LXDE, Ultra and Razor-Qt can make a problem.
To do that I used to use Lxappearance.
It saves new settings in 3 files:
If Lxappearance changes the first two files well, it does not change the last one “desktop.conf”.
So edit the “desktop.conf” file with a text editor and change the line from:
and save the file.