By "change this style" I am guessing that you mean that you want to update it to phpBB 3.3.
You have three options:
1. Apply all the changes from 3.0.14 to 3.3.4 which will be something in excess of 20k core code changes.
2. Create a new style based on prosilver that matches your 3.0.14 style.
3. Search for a new style that resembles your current style.
David Remember: You only know what you know and - you don't know what you don't know!
I now no longer support any of my extensions but they will start to become available here
eeji wrote: ↑Sun May 16, 2021 11:45 am
Number 2 from David's suggestions above would be your best bet, it's a lot easier to recreate an old style than update the original.