Signature colours not inverted

[kevprice83]

I've noticed that signatures are not being inverted to white as they are in the builtin Dark theme. This makes it quite difficult to read using the arc dark theme.

Dist: Fedora 27
Package: x86_64_.rpm

[kevprice83]

I can see in the dev tools the following style applied to the b element in the signature:

element.style {
    color: rgb(0,0,0);
}