It is tricky, never done it but just replicated your situation. You always end up selecting Page 1 instead of the Tab Control right - so the tab style never appears as a property?
OK, click so that your properties are showing properties for Page 1, then look at the bottom left corner - the black square for selecting is a little bit above the edge. Click beneath it, but still on the page (rather than the form) and your properties for the tab should appear and you can change style back.
Failing that it should be possible via VBA but the above works for me.