Add transparent logo in WebM format (for dark mode)

[basnijholt]

Description

Makes the contour around the animated logo transparent.

Currently it looks like:

Checklist

• Fixed style issues using pre-commit run --all (first install using pip install pre-commit)
• pytest passed

Type of change

Check relevant option(s).

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• (Code) style fix or documentation update
• This change requires a documentation update

[codecov-commenter]

Codecov Report

Merging #356 (1720286) into master (353b31e) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #356      +/-   ##
==========================================
+ Coverage   80.67%   80.75%   +0.07%     
==========================================
  Files          38       38              
  Lines        5071     5071              
  Branches      948      948              
==========================================
+ Hits         4091     4095       +4     
+ Misses        835      833       -2     
+ Partials      145      143       -2     

Impacted Files	Coverage Δ
adaptive/learner/learner1D.py	90.50% <0.00%> (+0.52%)	⬆️
adaptive/learner/average_learner1D.py	79.55% <0.00%> (+0.74%)	⬆️

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