Re-enable configurable background transparency support

[neiljp]

In #378 we allowed the terminal to set the background color, to support transparent terminals.

However, due to the range of terminals and customization available, it is likely better if this is an option rather than assumed by default. For example, some users report that with the default scheme then the background color looks strange - this is likely the default background color from the terminal, and we likely want 'black' instead, unless the user specifically requests otherwise (for transparency)

We would want to enable setting this option in the config file and on the command line, as with other options.

UPDATE: Transparency support was lost in a recent theme data restructuring. This feature therefore needs adding back, through a configurable option (command-line, config) which likely should default to off.

[mregdos]

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[mregdos]

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