Yes, the current dark theme does not have high enough contrast. There is a bug opened to switch to higher contrast colors in the dark theme here: https://bugzilla.mozilla.org/show_bug.cgi?id=971418 (and this will likely be uplifted to Aurora after landing).
FWIW, I'm colorblind and I use and prefer the dark theme.
Also, (warning this gets a little hand wavy) most pages on the web are light, so the light theme blends in. It is useful to have that contrast by default. Easier to make a distinction visually, and also "self xss" is deterred a little because the devtools are more obviously not part of facebook or whatever.