If you apply a theme to a sert of movie clips you can usually undo the theme by hitting the undo button at the top right of your project window or by hitting >ctrl + Z. But once you close and re-open your project, that option is not available any more. You could check your Auto save folder inside your project folder and see if you have a previously saved version before you applied the theme. This will of course only be posible if you have auto save turned on.
As I do not use Themes/Instant Movies much (mainly just to experiment for this forum), others will have to "fill in the blanks." However, one can Break Apart an Instant Movie, so that the various Effects, and elements can be adjusted, altered, or individually removed. This is somewhat similar to Breaking Apart a SlideShow transfered from PSE to PrE.
Themes are the application of a set of Effects, often some Audio, etc., and when Broken Apart, those elements can be edited.
To find the location, operations, and limitations of Break Apart, I would search the Help files for that term, "break apart," and probably under "themes," or "instant movies." In the meantime, I would expect that others will be able to point you directly to that command, and tell you exactly what you can do.
I can also anticipate that the only real issue would be where the Theme has applied Keyframes that ramp up/down over the Duration of a Clip. One might have move those Keyframes too, if they alter the Duration of a Clip. This would be done from the Effects Control Panel, accessed by first Selecting a Clip and then going to Edit Effects.
Good luck, and Happy New Year, plus welcome to the forum,