Shields is no longer dark

[cezaraugusto]

brave/brave-core#1931 changed Shields UI from always-dark to always-light. We want to make sure that this reaches release without this theme change. Either make it always-dark or, preferrably, make it switch depending on browser / OS theme-type choice (dark or light).

Test Plan:

1. Go to chrome://settings/appearance
2. Switch theme
3. Change active tab to an http(s) site
4. Open shields

Expected: Shields should match whether dark/light theme was selected in Settings
Actual: Shields is only ever light

[btlechowski]

Verification passed on

Brave	0.64.60 Chromium: 74.0.3729.91 (Official Build) beta (64-bit)
Revision	03844ed83e02b8add3f4b9cb859a7108d55b2e4d-refs/branch-heads/3729@{#860}
OS	Windows 10 OS Build 17134.523

Used test plan from the description.

Verification passed on

Brave	0.62.50 Chromium: 73.0.3683.86 (Official Build) (64-bit)
Revision	f9b0bec6063ea50ce2b71f5b9abbae7beee319a6-refs/branch-heads/3683@{#858}
OS	Linux

• Verified the STR mentioned in the description

Verified passed with

Brave	0.64.62 Chromium: 74.0.3729.108 (Official Build) beta(64-bit)
Revision	daaff52abef89988bf2a26091062160b1482b108-refs/branch-heads/3729@{#901}
OS	Mac OS X

• Verified STR from description