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Where can I find User StyleSheets or add a Custom StylSheet
I want to change how some html elements are displayed on my browser.
Use an extension. I can recommend Tampermonkey - Microsoft Edge Addons which gives untold opportunities. Otherwise, you can apply a custom style sheet by using a bookmarklet. Here's one with a single style (text-decoration:underline !important) that you can add to at will.
That is not an answer to the question that was asked. The OP is seeking information on how to add a user stylesheet, not how to modify the style of a single page.