Add the necessary colors and/or alpha components to the theme instead.
Also switch the background for ordinary channels to use the likely
intended theme option, instead of the window background color.
The general rule is that the channel color is drawn full strength in the
channel list on the left, and with alpha in the actual key frame area on
the right. This alpha is also reused with bone group colors.
Reviewers: brecht, billreynish
Differential Revision: https://developer.blender.org/D3813
eaa527f694