[Theme: Mobile] css editor cuts off at 99

Hi
Wordpress – latest
Theme – Mobile

I’m trying to do some css buttons, and I use the custom css editor to add the extra classes.
The editor truncates the file to 99 lines. Is this just the editor – if I edited the file with a text editor would it be OK or is this a limitation of the custom css file?

Where would I find the file to edit with a text editor?

Is there a better way to add a big chunk of css? When I had a try at making a child theme previously (not with this theme) it didn’t import the extra css file.

Thanks
David