Hello there,
If you are talking about the framework, you can change the presets there, if you want to override any file, please go to templates > your_template > html and rewrite the code inside the folder you want to change the code of. However, if you want to see the default files, go to plugins > system > helixultimate > overrides and you will find the code there.
Let's now talk about SP Page Builder. You will find all the codes of Page Builder inside components > com_sppagebuilder. You can change anything if you want to there, not to mention under no circumstances, it is recommended by us. Rest is upto you.
About adding CSS, you can do that wherever you want to. Inside every page, there is option to add CSS. Use the class name where you want to add CSS and add the code inside it, it's that much simple.
I hope you can understand.
Thanks!