Sorry, without access to the website I can't help.
You need to add:
Inspect the block to find the class of that block. (Go over the block > Right click > inspect element) then change .classOfThatBlock with the class you've just found.
If you click on “share preview” (bottom bar) you can send me the link of your “not yet published theme” so I can inspect the right element and solve your problem in no time.
But yes, that’s the file where you should paste the code once you’ve found the right class name.
Paste this code at the very bottom of your theme.scss.css file
let me know if it works!