Brave on Linux has no rounded corners with GTK theme

[ahydronous]

Description

GTK theme does not have rounded corners

Actual result:

Screenshot of Brave with GTK theme activated. Note the sharp corners of both the context menu and main window frame

Expected result:

Screenshot of Gnome Files. Notice the rounded corners of both the context menu and main window frame

Reproduces how often:

Always

Brave version (brave://version info)

1.61.104

Version/Channel Information:

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

[rebron]

cc: @fallaciousreasoning @zenparsing