Fleet UI: Update activity feed colors

[RachelElysia]

Issue

Cerra #24495

Description

• Update colors of dashes and timestamp on dashboard activity feed and host activity feed

Screenshots of fixes

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Manual QA for all new/changed functionality

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.56%. Comparing base (7ecd6d9) to head (0607ae9).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #24655   +/-   ##
=======================================
  Coverage   63.56%   63.56%           
=======================================
  Files        1600     1600           
  Lines      151545   151545           
  Branches     3893     3948   +55     
=======================================
  Hits        96334    96334           
+ Misses      47551    47550    -1     
- Partials     7660     7661    +1     

Flag	Coverage Δ
frontend	53.89% <ø> (ø)

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