Add theme Transparent balanced

[zoodogood]

Based on transparent_dark, adaptive for light scheme

[vercel]

@zoodogood is attempting to deploy a commit to the giscus Team on Vercel.

A member of the Team first needs to authorize it.

[laymonage]

Hey, thanks for submitting this. Unfortunately, it doesn't seem to have enough contrast when used with a light background:

It might be tricky to find the shades of grey that work well for both dark and light background, though...