Support styling across termdash

[mum4k]

Currently termdash mostly supports running on black background. Individual widgets can choose colors of elements on their canvas, but there isn't a good way to change the color of the entire application.

Additionally we could consider adding some effects similar to those used by the Linux dialog application.

apt-get install dialog
cd /usr/share/doc/dialog/examples
./form1

This is one example for inspiration:

[tcurdt]

I am currently searching for a way to set at least the background color.

[mum4k]

Hi @tcurdt and thank you for you interest in termdash. Going to address your question about the background color in #326.