Hi Dina. I'm not support аgent (just in case).
Try to make such actions:
Turn ON production mode in SPPB settings.
Clean Joomla and browser cache.
To know, read Helix documentation about what to use Custom CSS Field for.
My explanation: Only for cases when you use several template styles and want to make small differences between them. For all other cases, use custom.css file
Create custom.css file following Helix documentation.
Transfer all your code from templates's Custom CSS Field to custom.css file.
Correct errors in your code.
For example

Add your entire further code to the custom.css file