How to change the buttons theme?

Hello!!
I installed this applet on my laptop, running Debian 12 Stable, running MATE 1.26.0, and, although it works really well, the default button theme is completely different from what I like, and I couldn't find any options in the applet settings to change the theme. 
I found the issue #1 and #15, are these instructions correct?
Thanks for developing this applet for the MATE desktop!!