no problem! i think multiple highlight colors for rendered markdown is possible with html tags, just like the plugin martinkorelic linked. but if you don't want to use those brackets, i think you could use autohotkey to add those tags when you use a certain shortcut, you can adapt this cool script for that. but they are displayed on the rendered side. i don't know how to do it for the editor.
for others.. i have no idea. there are couple themes that comes with joplin though.
edit: i remember seeing examples of sidebar customization. there are probably some in the share your css topic.