Improve image drop zone contrast ratio

[Rahmon]

Change the color from white to dark gray ($dark-gray-700). Now the color constrast ratio is 6.16:1.

#2134

[karmatosed]

I think a better approach would be darken the blue and keep the icon white. Can you try that please?

[Rahmon]

Change background to dark blue:

[codecov]

Codecov Report

Merging #2767 into master will decrease coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2767      +/-   ##
==========================================
- Coverage   33.69%   33.42%   -0.27%     
==========================================
  Files         185      188       +3     
  Lines        5594     5639      +45     
  Branches      976      981       +5     
==========================================
  Hits         1885     1885              
- Misses       3141     3181      +40     
- Partials      568      573       +5

Impacted Files	Coverage Δ
editor/modes/visual-editor/index.js	0% <0%> (ø)	⬆️
editor/sidebar/table-of-contents/index.js
editor/sidebar/table-of-contents/item.js
editor/word-count/index.js	0% <0%> (ø)
editor/document-outline/item.js	0% <0%> (ø)
editor/document-outline/index.js	0% <0%> (ø)
editor/sidebar/document-outline-panel/index.js	0% <0%> (ø)
editor/table-of-contents/index.js	0% <0%> (ø)

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[karmatosed]

Looks good to me, thanks for doing that change. I'll merge it.