You surely have read Install.
And thus you know that before installing any modification, it is good to make a back-up of the whole Logic app in its default (Apple) state. A good way is to use the command “compress” (in the right-click menu), that is, make a zip of it.
Also, before installing your first new theme, make another back-up, this time of the default Resources folder.
After these steps you actually have two back-ups of this folder. The first one is included in the zip. Do not touch the zip. Move it to a safe place. Perhaps a cloud.
Now install a theme. Drag the downloaded new theme content (in the downloaded “Resources” folder) to the original Resources folder. Answer “replace”.
Now you are ready to use the theme.
When you want to try another theme, you have two options:
A) Find your backed-up (= copied Apple-default) Resources folder and copy it. Now you have again a fresh copy of the Apple-default Resources folder.
Drag the downloaded files (from the downloaded Resources folder) to this fresh folder… answer “replace”.
Then, use this folder to replace the Resources folder that you were using lately (= in the last theme).
B) Or, alt-drag (=copy) the files in your backed-up (= copied Apple-default) Resources folder to the Resources folder you were using lately… answer “replace”.
Drag the downloaded files (from the downloaded Resources folder) to this folder… answer “replace”.
With both ways (A & B), you are adding the new theme content that you just downloaded to a Apple-default-like Resources folder. That is a must, indeed. Add the downloaded content to the default setting, not to a modded theme setting.
If something goes wrong, you still have the whole app (including the default Resources folder) backed-up as a zip.
PS. The latest one for 10.0.7: Lemoncel Lite X V2. https://logicxinterfaces.wordpress.com/2014/05/16/themes-updated-to-10-0-7/