Set logo to have a white background for dark mode

[SeanNaren]

Adds a white outline to the lightning flash logo which is only visible in dark mode on github.

Now it would pass a contrast checker, helpful for accessibility :)

Before:

After:

cc @phoebynaren who sent this to me :)

[codecov]

Codecov Report

Merging #757 (081b110) into master (eaa0e96) will decrease coverage by 4.67%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #757      +/-   ##
==========================================
- Coverage   89.65%   84.98%   -4.68%     
==========================================
  Files         198      198              
  Lines       10681    10681              
==========================================
- Hits         9576     9077     -499     
- Misses       1105     1604     +499     

Flag	Coverage Δ
unittests	84.98% <ø> (-4.68%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
flash/text/question_answering/model.py	29.16% <0.00%> (-61.81%)	⬇️
flash/graph/classification/model.py	38.70% <0.00%> (-61.30%)	⬇️
flash/text/question_answering/data.py	28.29% <0.00%> (-57.88%)	⬇️
flash/graph/data.py	45.00% <0.00%> (-50.00%)	⬇️
flash/graph/classification/cli.py	42.85% <0.00%> (-50.00%)	⬇️
flash/text/classification/cli.py	57.14% <0.00%> (-42.86%)	⬇️
flash/text/question_answering/cli.py	58.33% <0.00%> (-41.67%)	⬇️
flash/text/seq2seq/core/utils.py	70.00% <0.00%> (-30.00%)	⬇️
flash/graph/classification/data.py	64.28% <0.00%> (-28.58%)	⬇️
flash/text/classification/data.py	62.92% <0.00%> (-28.09%)	⬇️
... and 7 more

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[ethanwharris]

LGTM 😃