RFPHelper console error when --toolbar-field-color is set to inherit

Fallout from #44214 (closed).

Get the console error:

RFPHelper: Missing color "--toolbar-field-color"

This is because this variable is set to inherit when using the system theme. The fallout is that --letterboxing-urlbar-text-color will be set to the wrong colour, but this isn't used with the system theme, so there is no visible consequences.

In principal, a custom theme could also set this to inherit. Regardless, we should at least clean up the console error.