Using inverting paint in viewports

[siber_cat314]

I just recently noticed the custom status-bar feature and was messing around with it. I wanted some of it to stand out, so I set the viewport's foreground to white.
And then I changed backgrounds and couldn't see it at all. So I thought to myself, "Gee, it would be nice to have an option to invert what's under those pixels to make it stand out against nearly everything."
Long story short, how hard would it be to support a "-1" or something else in viewport to use inverting paint instead of a single solid color. I'm willing to try coding this, but I don't know where in the source to go.

[JdGordon]

there are already 2 magic colour values, so adding another shouldnt be too hard (look in firmware/drivers/lcd-*.c)

[raudonkepuraite]

Gee, I dreamed about this long time ago. Also it would be nice to be able to force foreground color on in places where backdrop is loaded... you get the idea?

Some time ago I was able to make a theme using inverted font and it looked really nice!
Here's a screenshot... take a look.