Add background color to titlebar on macOS

[jlobos]

before:

after:

[keegancsmith]

Setting the title bar colour isn't in the spec. So I went looking for examples in other dracula repos and only found vscode (which is regarded as spec compliant):

https://github.com/dracula/visual-studio-code/blob/master/src/dracula.yml#L324-L328

In that case it uses different colours depending on if a window is active or not. Kitty doesn't support that.

Additionally this is the documentation for macos_titlebar_color:

Change the color of the kitty window’s titlebar on macOS. A value of system means to use the default system color, a value of background means to use the background color of the currently active window and finally you can use an arbitrary color, such as #12af59 or red. WARNING: This option works by using a hack, as there is no proper Cocoa API for it. It sets the background color of the entire window and makes the titlebar transparent. As such it is incompatible with background_opacity. If you want to use both, you are probably better off just hiding the titlebar with hide_window_decorations.

This sounds like an option which would negatively interact with the popular option background_opacity. As such I think we should avoid adding it to the theme. WDYT?