From vsTASKER, select the application title bar, right click, select Available Styles and choose the one you like from the list. The new skin is applied immediately.
If you like it more than the default WEB2 skin, remember the name (from the list), then open the default.db file in vsTASKER root directory, and replace the skin name (WEB2 default) with your prefered one (name).
If you prefer not to use any skin at all, select Tools::Preferences::Systems then Forms, and in Application Skin box, choose Windows Default.
Save and restart vsTASKER.