We are aware of the problem that a small % of forums have who use a white logo. While I really appreciate how annoying this is, it’s impossible for the default to “work” for everything.
There isn’t exactly a way to do in settings, but your best bet is to indeed use custom css. There is a class anchor for the signup pages:
.signup-page,
.login-page {
.d-header-wrap {// colour}
}
Let me know if this works for you!