Style not being applied to newly installed themes

It seems that the stylesheet are not linked properly.

If you are using Google Chrome, right-click and select Inspect. A new window with all kinds of code will show up. Find and expand the <head> element. There, you will find a long list of elements. These are the stylesheets enqueued from a wide variety of sources. Check whether they have identifying ID or not.

If they haven’t, then it’s the cause of your problem.