If the first solution does not work in the list below please test one of the next two options:
A) Manually change skin by going to File->Skins, change to Classic or Black;
B) Press Windows +R keys on keyboard to open the Run dialog box, enter regedit in the Open: box and select OK to open Windows Registry Editor. Then browse to the following key chain to delete it: "HKEY_CURRENT_USER\SOFTWARE\Foxit Software\Foxit PDF Editor 11.0\Preferences\Skins".
Then re-launch Foxit PDF Editor;
C) Uninstall and Re-install the software