Implement support for equilux theme

[RedFantom]

With the introduction of the equilux theme in the ttkthemes library, the GSF Parser could offer a setting allowing users to switch between a dark and light theme (much like the theme setting introduced and phased out somewhere in v2.x.x development).

If only the set_theme function call is changed various issues arise:

• Some Treeviews look a little wonky (this is to be addressed in the ttkthemes repository)
• The Calendar widget day name row has extremely low contrast
• The blue text does not offer enough contrast on the dark background
• The background color of Canvases must be adjusted (the graphs are also a Canvas)
• Some sort of text size adjustment is required (at least for the Treeviews)