Change text-muted value and bind color to body-color

Created by: Psixodelik

Hi

I've been doing a lot of website accessibility lately. I am very happy that Bootstrap provides a lot of possibilities for ensuring correct accessibility.

But I've been worried about the text-muted class for a long time. Right now, it is just a gray color, which has nothing to do with the default body-color.

Because of this, WCAG color contrast is only provided against a white background.

I suggest using the ready-made tint-color function and defining the text-muted color by mixing the body-color and white color.

You end up with something like this:

tint-color($body-color, 30%);

What do you think of it? If necessary, I will do a PR