Buttons should use Brave brand style

[petemill]

Chromium webui pages all use material color palette for checkboxes. These should use brave color palette.

Test plan

Check a checkbox on the left of any history item in chrome://history and observe the buttons on the top for 'cancel' and 'delete'. They should be rounded with a black border and text.****

Old

New

[LaurenWags]

Verified passed with

Brave	0.63.21 Chromium: 73.0.3683.75 (Official Build) dev(64-bit)
Revision	909ee014fcea6828f9a610e6716145bc0b3ebf4a-refs/branch-heads/3683@{#803}
OS	Mac OS X

• Verified test plan from description
• Verified test plan from WebUI: Navigation Bar and other webui overhaul brave-core#1422

Verification passed on

Brave	0.63.20 Chromium: 73.0.3683.75 (Official Build) dev (64-bit)
Revision	909ee014fcea6828f9a610e6716145bc0b3ebf4a-refs/branch-heads/3683@{#803}
OS	Windows 10 OS Build 17134.523

• Used test plan from description
• Used test plan from WebUI: Navigation Bar and other webui overhaul brave-core#1422

Verification passed on

Brave	0.63.20 Chromium: 73.0.3683.75 (Official Build) dev (64-bit)
Revision	909ee014fcea6828f9a610e6716145bc0b3ebf4a-refs/branch-heads/3683@{#803}
OS	Linux