I can’t help you with the @import thing… but most text editors have code snippets, maybe it’s a suitable alternative to manual copy/pasting for you ?
in Atom, you can create snippets to quickly reuse code (type aprefix + tab to auto-paste a multiline block) : https://www.hongkiat.com/blog/add-custom-code-snippets-atom/
It works with vscode, pspad, sublime, notepad++ (pick this one and NppSnippets/quicktext plugins if you want a lightweight portable solution), etc… alternatively, try a text-expander ?
TL;DR
use code snippets in atom, File>snippets :
'.source.css':
'AnyTitleLikeAndyMode':
'prefix': 'Andy'
'body': """
....paste your snippet here between triple quotes....
"""
next open you obsidian.css, type `Andy’+tab (or whatever prefix you defined) and it will paste the rules you defined in the snippet.
Not as streamlined as @import but better than ctrlC ctrlV