Add template for leftwm theme.ron

[VuiMuich]

As suggested in PyWal #688 here is a template for creating a theme.ron.
It was tested a couple of weeks ago with original pywal, except the background_color which is a new feature that is expected to become merged by the end of the week.
@arcolinux could you give this another test spin?

Edit: also worth noting, the background_color will go unseen in almost any use cases, but I thought it might make sense to style all available color fields.

[eylles]

i can merge it to master right now, but if you want i can wait until @arcolinux @arcolinuxz reports back from the test you know for sanity's sake.

[arcolinuxz]

I let someone else test it. Just added the one file in my /usr....
My first attempt was not working.
I have no time to retest it soon.
At the soonest in 10 days or so

[eylles]

welp, will hold onto pushing the release of pywal16 until sunday to give this some days and look for anything else that may need to be cleaned up before releasing, if this template has not been tested by then i will just merge it and any problem that appears can be addressed in a late bugfix release.

[eylles]

into master this goes.