Update theme 50 scales to align with sidebar background usage

from: gitlab!136646 (diffs, comment 1645147247)

Here's some previous discussion and context to why mix() has been used here gitlab!129958 (comment 1523080013)

HEX values

Theme super-sidebar-bg palette-50
Indigo #F8F8FF #F1F1FF
Light Indigo #F8F8FF #F1F1FF
Blue #E6ECF0 #CDD8E3
Light Blue #EEF3F7 #DDE6EE
Green #EEF4EF #DDE9DE
Light Green #EEF4EF #DDE9DE
Red #FAF4F3 #F4E9E7
Light Red #FDF9F8 #FAF2F1
Gray #F6F6F7 same
Light Gray #FBFAFD same
Dark Mode (alpha) #1F1E25 same
Edited by Scott de Jonge