I have had these errors in the past (support ticket #49129 ). But allthough these errors should not happen I still could use the theme and adjust it to my needs. Could you elaborate why it is preventing you to customise the custom.css file?
And file s support ticket anyway. The theme creator should not create themes which generates errors.
I had those errors once when my theme tried to import fonts with url 'false'. This was some mixup with the theme creator when you did not specify extra fonts, but only used the default. I think I read somewhere that it should be fixed.
Anyway, check your sass files for the typography section. I commented out the font familiy import section, because I did not need to include extra fonts.
//## Change your font family, weight, line-height, headings and more (used in components/typography)
// Font Family Import
I found a workaround for this kind of errors. Error is caused because of non existing classes that appeared in mendix theme lib.css file. To be more precise error occurred because of missing:
I copied all content from the lib.css from mendix theme file to standard default mendix lib.css file and I removed those above mentioned two lines of code. After that everything worked correctly. Now I have a styling that I created in mendix theme creator and I don't have mentioned errors that prevented me from using custom created css.
I hope that this will help someone :)