Depends on what classes you used in your custom styles. If you right-click on the new window, you can see that only a few classes are used, not the usual tc.tiddler-frame, tc-tiddler-view-frame, etc...
You'll want to add to your existing styles the
tc-single-tiddler-window-based or the
tc-page-container-based classes to match if you have anything based on tc-tiddler-frame or similar.