FIX: use new color variables in embed.scss, remove hardcoded colors (PR #10898)

My previously merged PR (https://github.com/discourse/discourse/pull/10887) was reverted (https://github.com/discourse/discourse/pull/10897) because I confused the path for color_transformations with the path for color_definitions — the SCSS in this PR is identical, but it uses the correct path.

GitHub

all passing, giving this another shot!