These "User Style Sheets" are added by the Chrome extensions declared in the manifest file , or using chrome.tabs.insertCSS.
Styles on a web page always take precedence over the user style sheet, unless the rule set in the user style sheet (extension) has a higher value.
If you want to know which extension is responsible for these style sheets, simply disable the extensions one by one until the styles disappear. After disabling the extension, select another item in the inspector, then select the original item to refresh the view, you do not need to reload the page.
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