Remove or change colour of window border?

I have modified the background colour of my installation to be a very dark blue, but there is still a border around the window (and the background of the tabs) that is white or light gray, which I find quite distracting. An example screenshot is at the following link:

https://i.imgur.com/VZEr8ma.png

Is there a way to remove the border (excl. the tabs), or to change it to be the same colour as the user-specified background colour, or even just have a 'dark mode' so that it is black and blends into the background?

Thanks, Luke