Converted hard coded shades of gray to variants of @gray.

I discovered while attempting to make a theme for Bootstrap that it seemed unnecessarily difficult to tint the color gray used throughout since there is a combination of `@gray-*` variables and hard set gray hex values used in the variables.less file. To fix this, I changed the `@gray` variable to be hard set to `#555` and used that to set the other shades of gray. I fixed all the places that should have already been using the existing variables and then lightened and darkened `@gray` in the remaining instances.

This does slightly shift the colors in a few outcomes, but each seemed very minor. While this does make it a bit more arduous to manage the various shades of gray, it makes the variables file much more useful for making sweeping changes in lightness, darkness, or tint by simply changing the `@gray` variable.