Add a dark theme for dev-channel

[Sa-Ja-Di]

As there are also many people around with some eye issues, a dark UI would help such people quite a lot.

That suggestion was also made on the Brave old repository, perhaps this idea can built inside the newly planned Brave, based on real Chromium?

[kekukui]

Developers, please reference the following comment which offers a potential solution:

brave/browser-laptop#271 (comment)

[Jacalz]

How do you switch back to the light theme? It just switched to dark theme when i updated and I couldn't find any option to change to light... cc @bbondy

[bbondy]

Option will be added but that's the default for dev channel only.

[btlechowski]

Verification passed on

Brave	0.55.5 Chromium: 70.0.3538.16 (Official Build) dev (64-bit)
Revision	16ed95b41bb05e565b11fb66ac33c660b721f778-refs/branch-heads/3538@{#306}
OS	Windows 7

Verification Passed on

Brave	0.55.6 Chromium: 70.0.3538.16 (Official Build) dev (64-bit)
Revision	16ed95b41bb05e565b11fb66ac33c660b721f778-refs/branch-heads/3538@{#306}
OS	Linux

Verified passed with

Brave	0.55.6 Chromium: 70.0.3538.16 (Official Build) dev (64-bit)
Revision	16ed95b41bb05e565b11fb66ac33c660b721f778-refs/branch-heads/3538@{#306}
OS	Mac OS X