Where did I put this one line of css?

You can use the Developer Inspector (usually F9 in your browser) to bring up the page data. Right click where you see the CSS being used, and use the developer panel to click on the CSS code (you should be on the Inspector tab).

Over in the right side of that developer area you should see the various CSS styles. And to the right of that, the Computed CSS.

Click on one of the items in the Computed area. You should see the file and line number where that value came from. Example below. See the CSS for the ‘border-top-right-radius’ came from ‘stacks.css’ in line 8.

Sample screen shot of developer area

The screenshot is from the FireFox Developer screens. There are many tutorials on how to use this that you can find on the googles/bings/ducks.