You can find the stylesheet here. And in your contao installation you can find it in the vendor folder
/root/vendor/terminal42/contao-fineuploader/src/Resources/public/frontend
You could look up all the selectors there but if you overwrite them there, all your changes will be gone after an update. So usualy you just overwrite them in a new e.g. 'my_styles.css' which you include directly in your fe_page.html5 or in your Layout.
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