Change (or allow to change) "invisibles" transparency

[Minkiu]

Hello,

thanks for the lovely theme, it's been my trusty companion for years now; there is only one thing that I modify every time I set it up.

I mostly code on Python so I find the Sublime Text setting "draw_white_space": "all" quite useful, but with the current colour setting for the "invisibles" it pops way to much, so I tone them down:
"invisibles": "color(var(predawn) alpha(0.25))"

Before:

After:

This could be a setting? I am not versed in sublime themeing:
"predawn_invisibles_alpha": 0.25

If you are happy changing the global setting I can do a PR, if it needs to be a setting, I'll have to investigate more!

Let me know,
Cheers.