Hi,
It's not a good idea to directly change in main Library, Lets say if newer version of CodeMirror available and we want to pull new version then we need to maintain this change set every time we pull new version.
Instead please use,
- web/pgadmin/static/css/overrides.css
or more specifically if you are only targeting Syntax highlighting for Query Tool then,
- web/pgadmin/tools/sqleditor/static/css/sqleditor.css
to override any CSS related changes specific to pgAdmin4.