Thank you yyuuiko - that is a nice well-organised extract from the asset folder. I wish I had seen it earlier.
I found the asset folder before posting my request, and tried applying the icons but they still don't look the same as the native icons. For instance, if I use the brightness icon in the BTT brightness slider widget, it is much smaller than the native brightness icon in the control strip. I played around the settings to see if removing padding or adjusting the icon height would help but it just distorted the icon and made it look worse.