rename the bsz file to .zip
extract it to a folder
edit the skin.ini file:
look for string "Btnx="
where x is 1,2,3 etc
you'll find things like
once you're done, you can either leave this as is (I mean all the skin files in the folder) or recompress as a zip then rename the zip file to .bsz
Hmm I already tryed it but it doesnt work for some reasons. If I change something in skin file then I cant use that skin anymore. It's like I rename the file test.bsz to .zip and extract all the files from there. Then i change something in skin.ini and save it. Then I compress it again and rename it to .bsz. Then I try to open it with BSP but it shows default skin and errors.
you should be careful about what you compress.
do not select the folder and compress it. Select the whole set of files inside the folder instead and compress them.
compressing the folder instead of the files is the mistake I made in the first place.
(I'm not sure what I write is very clear :| , tell me if you want me to try once more ;) )