There are several ways to add code box in Blogger blog posts. Many of them would suggest to edit blogspot template and add third-party CSS links. But I prefer a way that doesn’t require any tampering or modification of blogspot template.
First of all, it is risky & you may corrupt template HTML. Secondly, if later you want to move out of Blogger & host it in some other blogging platform like WordPress, that will break the formatting of the code block. You would have to add these CSS links in new template manually.
If you use proper HTML code without any third-party links for code display, that would be consistent & won’t break even if you move out of Blogger platform.