Easy transparent background support

[Kafva]

I would prefer it if the background could be made transparent.
This should be possible via a theme but I personally find the configuration file + theme file a bit to cumbersome to setup, would prefer a simple command line switch, e.g. --bg-transparent.
The drop down menus ('Analysis' and 'Misc') need a different color from the default to remain readable in this setup (not sure if this is currently supported for themes).
I have a hack that enables this behaviour (hardcoded, no command line switch), see patch, would be nice if it was officially supported.

termshark-transparent-background-for-dark-mode.patch