WebUI Navigation Bar sub-menu has light-on-light items in dark mode for c74

[petemill]

WebUI Navigation Bar sub-menu should be on dark background in dark mode
C74 only

Test plan / STR:

1. Visit Bookmarks Page

Actual:

Expected:
Dark background

[bsclifton]

Resolved when C74 was merged to master with #3931 😄

[kjozwiak]

Verification PASSED on macOS 10.14.5 x64 using the following build:

Brave	0.65.113 Chromium: 75.0.3770.38 (Official Build) beta(64-bit)
Revision	3860105745f2b12537da9e9f048f14c3f52ba970-refs/branch-heads/3770@{#618}
OS	Mac OS X

• Example of chrome://bookmarks/:

• Example of chrome://downloads/:

Verification passed on

Brave	0.65.113 Chromium: 75.0.3770.38 (Official Build) beta(64-bit)
Revision	3860105745f2b12537da9e9f048f14c3f52ba970-refs/branch-heads/3770@{#618}
OS	Linux

Downloads page

Bookmarks page

Dark/Light theme has no change on Linux.

Verification passed on

Brave	0.65.114 Chromium: 75.0.3770.38 (Official Build) (64-bit)
Revision	3860105745f2b12537da9e9f048f14c3f52ba970-refs/branch-heads/3770@{#618}
OS	Windows 10 OS Version 1803 (Build 17134.765)