I don't think that is what is intended.
just create a copy of prosilver, rename the folder and change the style name in the style.cfg file.
then replace the default icons/images with yours.
then delete all of the files and folders from the templates folder.
then upload and install this new style just like any other style.
then set it to be used by whoever you wish.
when the next udpate comes along simply do the full replacement update method and manually edit any of the css files in the theme folder of your child style.
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Premium phpBB 3.2 Styles
OK, so what's the speed of dark?