Unless you are VERY experienced with Mathematica styling I wouldn't recommend writing your own style files.
A common practice (which is fast) is to make local changes. You can later copy these changes to new notebooks by using SaveAs and deleting the old content, or copy the modification you have made and paste them to a fresh notebook
You will find that when Mathematica changes version and you reinstall all "your" style file "vanish" and you will have to put them back (if you are lucky enough to save them elsewhere) on the new Mathematica system.
Usually, this does not worth the bother, and the above "pragmatic" way is suitable for many cases