prosilver and subsilver2 come with package and as they are created by phpBB every one knows about its license.
You Will notice that prosilver_se also have license.txt file .
There is no such basic format for the license file the only thing is it should be complete and not self modified.
For example you select GPL V2 as you licese so you will be adding GPL v2.0.txt
with your styles.
Yes if you use GPL 2.0 for your all styles then license.txt file for each style will be identical.
License.txt file is added to the package so that the user who is using gets aware of the license of the style.
Style Team will be soon making some announcement about that but i am not sure when.