fix(GlModal): Fix modal backdrop color in dark mode

What does this MR do?

Fixes gitlab#353021 (closed)

Use correct variable name for modal backdrop bg - https://github.com/twbs/bootstrap/blob/v4-dev/scss/_variables.scss#L965

This was previously inverted in light mode, so modal backdrops were light colored.

Note that on gitlab gl-modals backdrop still looks extra dark because we have 2 of them. We have .modal-backdrop, and also some legacy CSS for .modal that I think we still need for remaining non-gl-modals.