Reported this 5 weeks ago whilst in beta, obviously wasn’t picked up!
In the url you can pass a fullscreen option which at least gets rid of the border and sizing issue.
/gui-v2/?fullscreen
There is a setting “Remote console appearance” setting in the new UI its settings → General → Display & Appearance menu? When set to auto, it will follow the browser preference. When set to “Same as GX display” it won’t. Make sure it is set to “Same as GX display” and GX display appearance to Dark.
Can you check which version you are running by accessing the service menu. Press and hold for 5 seconds the bottom left corner of the display, then a side menu is opened, at the bottom of that menu the app version is shown. Latest version is v1.01.