The proper way to do this is documented here: https://docusaurus.io/docs/api/themes/configuration#navbar-logo-srcDark

I also used Ismail Vittal's fix. I didn't find any documentation about how to do this otherwise. Alexey Pyltsyn did the (un-referenced) update to introduce this functionality include a documentation update?

I couldn't use the https://docusaurus.io/docs/markdown-features/assets#themed-images approach in the docusaurus.config.js so I ended up using Ismail's fix.. Thanks!

Quick CSS fix: (If specific to Docusaurus)

html[data-theme='dark'] .navbar .navbar__logo {

content: url(img/dark-logo.svg)

}