Hi @Deuxv3, your question involves a mixture of different areas. To customise Obsidian, there are a few areas to be covered. You can mix them or play with them one by one.
But the most important thing is that once a plugin is in your vault and installed, you can’t (or, more complexly, it’s almost impossible) edit it. Plugins can be edited in source code (usually GitHub), compiled, and installed.
So …
CSS snippets: CSS snippets - Obsidian Help
CSS snippets are the easiest things to play with. It’s not suggested that you change any original CSS file from Obsidian or any plugins. All of them are using them, and any changes can bring them to the point that they will stop working. So, as the link suggests - create a new CSS file and start customizing it there.
Plugins: Build a plugin - Developer Documentation
I do not know how familiar you are with coding/programming, but if this is your horse, dive into the beauty of developing and modifying plugins. Take a look also of just trying community plugins - Community plugins - Obsidian Help
Themes: Build a theme - Developer Documentation
This area is big! If you want to build a beautiful, useful, and user-friendly theme, you need to dive deep. Check some themes - Themes - Obsidian Help
Some plugins enable you to play with TypeScript and JavaScript inside Obsidian without building plugins.
I know it’s boring, but Obsidian’s Help is a good starting point for familiarizing yourself with options and then with the next steps - building those options.
I hope you get something to start.
Cheers, Marko 