h3o in the theme location, there is a folder called "gtk-3.0". Inside it, you will find 2 files
gtk-dark.css. In fact, you can edit both of them, but I just did the trick for the dark one (because i am using dark mode theme). So open the file you want and just change replace the color hashtag for the one you want in those lines:
@define-color budgie_tasklist_indicator_color #d4d4d4;
@define-color budgie_tasklist_indicator_color_active #d4d4d4;
Here we go! Now, you have a fully clear dock panel aha!