gl-text-subtle and super-sidebar-mix-blend-mode in app/assets/javascripts/super_sidebar/components/pinned_section.vue

Follow up from !170573 (comment 2223516687)

Implementation Plan

Investigate super-sidebar-mix-blend-mode class to see what it is doing and consider removing it if it is redundant. Make sure to read the linked comments and feel free to ask for help on the comment thread if you aren't sure what to do.

Please add screenshots of the component before and after so that it is easier to review and validate this change

Added a weight of 3 because these classes can be confusing and get complicated quickly (and I dont really understand what needs to be done tbh)