If you want to make the changes directly in the theme.css file, since the current formatting is highly inconvenient, you can simply use a CSS formatter to format the CSS into a usable format, and then use the find feature to make changes. However, I highly recommend making changes in the custom.css file as it will be easier to keep track of the changes you make and alter them easily.
I recommend using this online CSS formatter and replacing the original CSS code with the output from the formatter: https://www.cleancss.com/css-beautify/
Edit: If you make changes to an element in the custom.css and the changes are not showing up, add "!important" before the closing semicolon on the line as illustrated in the attached picture.
