Some icons may not fit and/or look right with a different theme. You may want to look into a freeware program called paint.net Many of the real themers on this site use it (how do you think I found out about it?)
https://www.getpaint.net/
Anyway, I've unpacked themes and used the toolbar_blank.png file as a starting point and created a lot of custom icons that fit my current theme. It's a bit tedious, and you may wonder why you're not just making music, but for spare time stuff it can be fun.