Hi! Thanks for your response!
It looks like the author of the skin hasn't updated or commented for a while so I decided to fix it myself.
However, I don't know how to change it to bmp like how you did and when I do (I use Make copy of file to make a .bmp, later on I saved a new BMP file in photoshop), it shows a arrows_glyph.bmp file that is surrounded by black background like this and applying this file made all the scrollbar button black also.
I still couldn't fix the problem, please help. Thanks!