the browse button is embedded in the <input type='file'>
to be honest, i tried quickly to make any change to that button, and found no way.
about adding style, i can suggest you to read this comment in the default template
Thanks to the following line, you can customize the appearance of the original template by
right click on the root -> advanced -> diff template
then type [style.user] followed by all the CSS commands you want.
This way you will stay up to date while the original template improves.
i just noticed this trick doesn't apply to all the pages like the upload page, but just to those for browsing.
i will correct this in next build.