Dark mode does not work for the new tab Remove menu

[cndouglas]

Test plan

See brave/brave-core#4423

Description

Follow up to #5014

The new tab Remove menu does not work with dark mode.

Steps to Reproduce

1. Open Settings and set Brave colors to Dark.
2. Open the new tab page and click one of the drop down buttons next to one of the new tab page elements.

Actual result:

The menu is light even when dark mode is enabled.

Expected result:

The menu should be dark, like the settings popup.

Reproduces how often:

Always.

Brave version (brave://version info)

Brave | 0.72.83 Chromium: 77.0.3865.90 (Official Build) nightly (64-bit)
Revision | 58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS | macOS Version 10.15 (Build 19A583)

Version/Channel Information:

• Can you reproduce this issue with the current release?
• Can you reproduce this issue with the beta channel?
• Can you reproduce this issue with the dev channel?
• Can you reproduce this issue with the nightly channel? Yes.

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? Not applicable.
• Does the issue resolve itself when disabling Brave Rewards? Not applicable.
• Is the issue reproducible on the latest version of Chrome? Not applicable.

Miscellaneous Information:

[rebron]

@cezaraugusto Can you take a look?

[btlechowski]

Verification passed on

Brave	1.7.65 Chromium: 80.0.3987.132 (Official Build) dev (64-bit)
Revision	fcea73228632975e052eb90fcf6cd1752d3b42b4-refs/branch-heads/3987@{#974}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#4423

Light theme

Dark theme

---

Verified passed with

Brave	1.7.66 Chromium: 80.0.3987.149 (Official Build) dev (64-bit)
Revision	5f4eb224680e5d7dca88504586e9fd951840cac6-refs/branch-heads/3987_137@{#16}
OS	macOS Version 10.14.6 (Build 18G3020)

• Verified test plan from NTP: Update hover colors for context menu  brave-core#4423

Dark theme:

Light theme:

Verification passed on

Brave	1.7.67 Chromium: 80.0.3987.149 (Official Build) beta (64-bit)
Revision	5f4eb224680e5d7dca88504586e9fd951840cac6-refs/branch-heads/3987_137@{#16}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified the test plan from NTP: Update hover colors for context menu  brave-core#4423
  Dark Theme