Hello theme help

Hi. I wondered please if someone could point a ‘blog noob’ in the right direction? I’m using the Hello theme. I’d like to set the default look to ‘light’ and not dark. I think I can achieve this through custom CSS? The themes author says add a config change to config.toml? Can someone please explain how I do this? Mmm… I have a feeling its because i’m only subscribed to micro.one and not micro.blog. Thanks a lot. - Richard.