Custom CSS field

I am working on a website for a group of researchers at a University. As it’s expected they have tight security on their servers so I can’t really access the WP files for my Origin child theme. I made my child theme and made all necessary customizations but I still need to overwrite certain things from the main Style Sheet that for some reason have not been overwritten by my child theme.

My files are not writeable and the IT team at the university won’t make them so.

I thought I’d make my modifications by adding Custom CSS but I find the little custom CSS field extremely difficult to work with as it is just a Text box.

Is there an alternative? Would you consider making it a text area in a future update? Right now I’m doing it the hard way, coding in Sublime text and pasting my modifications in that little text box but It’s a pain.