Monica did a nice job with her themes.
CPG 1.6 has features built in that make it easier for simple themes to do much more with their CSS. For instance .. the CSS can know which type of page it is on and not display the full banner at the top of the page for album thumbnails or intermediate images. Or don't display the full menu on some page types. I would add modifications to the CSS of that theme to take advantage of those concepts.
I would like to have an advanced level of knowledge about php and css, but I am not able to do anything other than change a color, or the size of a font, font, etc. I dedicate myself to see the css code and how it affects the page and modify what I can understand. I guess with time I will learn more things

You can disable an individual plugin to see whether it may be causing a particular issue that you are seeing. If you identify an issue with a plugin (or theme, for that matter), you should first try to message the plugin (or theme) author to see if they are still supporting their work.
CPG does not show me the plugin manager with PHP8.1, I get the following message:
Fatal error: Uncaught TypeError: in_array(): Argument #2 ($haystack) must be of type array, null given in /galeria/plugins/minicms/configuration.php:38 Stack trace: #0 /galeria/plugins/minicms/configuration.php(38): in_array() #1 galeria /pluginmgr.php(146): include('/home/u41263082...') #2 /galeria/pluginmgr.php(725): display_plugin_list() #3 { main} thrown in /galeria/plugins/minicms/configuration.php on line 38
The only solution I see is to downgrade to PHP7.4, disable one of the plugins, upgrade back to 8.1 and see if anything has been resolved. If it is MiniCMS or BB Control, try to contact the author, but I think they are no longer operational. Perhaps you have no choice but to stick with PHP7.4
Ever since I have been involved with CPG, I have disliked the filmstrip ... especially how it was implemented. I have avoided doing work on it. I would rather see it gone from the core of CPG. It could better be implemented as a plugin for those who find value in the concept.
I totally agree with you, it would be more interesting to leave the filmstrip as a plugin for those who want to implement it. However, I like how it looks, but aesthetically it is more interesting without the filmstrip_background, for that reason I removed it from the chaoticsoul theme.
I will try to contact Monica, the truth is that her themes are very good, I hope I can buy her a more current theme that also works with php8.0