[themes] Update the sidebar and accordion colors for the dark theme (…

[gabrielluong]

…#3909)

Associated Issue: #3909

Summary of Changes

• In the dark theme, Sidebar background color is var(--grey-90) and Accordion header background color is #141416.
• In the sources list, and add watch expression button, the background color needs to be var(--theme-sidebar-background).
• The accordion header needs to be #141416 only in the dark theme and var(--theme-toolbar-background) in the light.

Screenshots

Before:

After:

[jasonLaster]

why the list and not the panel?

[gabrielluong]

Fixed.

[codecov]

Codecov Report

Merging #3912 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3912      +/-   ##
==========================================
- Coverage   55.75%   55.73%   -0.03%     
==========================================
  Files         129      129              
  Lines        4902     4904       +2     
  Branches     1014     1015       +1     
==========================================
  Hits         2733     2733              
- Misses       2169     2171       +2

Impacted Files	Coverage Δ
src/components/Editor/EditorMenu.js	0% <0%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #3912 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3912      +/-   ##
==========================================
- Coverage   55.75%   55.73%   -0.03%     
==========================================
  Files         129      129              
  Lines        4902     4904       +2     
  Branches     1014     1015       +1     
==========================================
  Hits         2733     2733              
- Misses       2169     2171       +2

Impacted Files	Coverage Δ
src/components/Editor/EditorMenu.js	0% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ad186e9...f09a708. Read the comment docs.