There is just a small issue that the css not better work with older browser like ie6 to show warning, as we know this plugin work is to warn people that they are using old browser so the warning must look better and at place in older browsers.
And I haven't found the file to do modification in css or main file of the plugin? where it get installed I am not able to see it in plugin system folder?
Overall its working fine and able to show all the download options and continue if people want.
The plugin css file can be found in /media/plg_system_browserupdatewarning/css/BrowserUpdateWarning.css
I wouldn't suggest editing that file though, as your changes would be lost if you installed a new version. I would suggest, however, that you copy the contents of that file into your template css file and make changes there. Then you can just turn the plugin css off in configuration.
I will do more testing for older browsers.