The dark theme (which I’m pretty sure just came stock with Discourse and hasn’t been tweaked) also is missing navigation styling and maybe some other css on desktop that the Crystal theme applies.
In general it would be great if the dark theme could be reviewed at some point to match Crystal.
We could provide Dark Crystal, even… 
A simple fix could be to put a white-circle background behind the “Crystal” in the logo – on the default theme, it’ll blend into the background, and on the dark theme it’ll make the logo visible.
I added a background property to CSS’s #site-logo
.d-header #site-logo {
height: 2.57em;
background: #ffffff45;
}
Which makes it look like:
Which IMO looks sufficient. But yeah probably fiddling around with the properties in #site-logo someone can come up with something better. I don’t know if that CSS only applies to the dark theme though
