part of the upgrade is to replace the 3.2 files with 3.3 files, so yes, you will need to redo your custom css in prosilver 3.3
having said that, it is usually not a good idea to edit prosilver since it will always be updated for each board update.
the usual way to do what you want is to create a copy of prosilver, rename the prosilver folder whatever you want.
if you are only editing the css then you can remove all of the files from the template folder and leave the css files in the theme folder.
then, edit the css as you wish and install the style like any other style.
then choose it as default and override users in the admin panel.
now, when you update in the future, you just do the full file replacement method and then you only have to manually update your custom style files.
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