I find it difficult to find full tutorials on the category override so maybe you could help me?

K2 default font family is different from my template font, so I need to do an override.

I have copied a template folder with a blog_style.css file (with applied font changes) to com_K2 folder but nothing shows up in K2 category template options. The remote site looks like this: /httpdocs/templates/ca_cloudbase2_j25/html/com_K2/templates/Blog

I have also enabled the Css4k2.

I have not changed any .php files. The blog_style.css is a tweaked cat_cloudbase2_j25 template file.
Thanks!

Your path looks correct. when you go to k2 categories, do you see the override at the top right?
please attach the css file

It doesn't appear in the Category Item Layout.
Attached is the blog_style.css file. As mentioned earlier, it's just the original template file where I replaced the font family - might be an error there?

The blog_style.css is a tweaked cat_cloudbase2_j25

was misleading in your post.
The file does appear to be k2 styling as you say.
Check the paths again

YourSiteRoot/templates/YourTemplateName/html/com_k2/templates/YourTemplateName directory should contain all the override AND the _style.css file

Yeah, that's the K2.css I am using (from the K2 folder). I obviously got the wrong understanding of that from tutorials, that seem to cover only the basics. Is there any detailed tutorials out there on creating the override and the style.css as it's difficult to guess being completely new to this system. The K2 tutorials leave a lot of room for assumptions and changing the font seems to take me way longer than it should.
Thanks for all the advice so far by the way!

Try the ostraining videos under documentation.
They demonstrate most things