If you are using this theme component, you will need to update it to the latest version: https://meta.discourse.org/t/alternative-logo-for-dark-themes/88502
@cookywook it looks like it’s working on your site now - let us know if you’re still having issues and I’ll be happy to take a look (I happen to be a Monzo fan as well as working for Discourse 
)
@DNSTARS let us know if updating the theme component works.