With the latest release I noticed that all of my custom panel are no longer colored. Not sure if I should flag this as an issue on GitHub yet because maybe the way this is done has changed. See the instructions I copied from the pyRevit docs for how I structured my yaml files.
Panel Background
You can set the background color for main, title, and slide out parts of a panel in the bundle metadata file. The colors are in CSS style and can be in ARGB or RGB format. Note that colors need to be wrapped in quotes since # is a comment character in YAML
# set background for all parts
background: '#ff0000' # or ARGB e.g. #ff0000ff
# or for individual parts
background:
panel: '#ff0000'
title: '#00ff00'
slideout: '#0000ff'