How to disable external CSS in browser for testing?

The easiest way is one of the below, or both, according to your personal preference:

1. Temporarily comment out the tag that links the stylesheet in your HTML by putting it between <!-- and -->.
   This is done automatically by some IDE's like Codium, when you press CTRL+/ while the cursor is on the line of the tag.
2. Make use of extensions like uBlock Origin and blocking the url's of the css file(s) you want to eliminate for your testing.