chore: apply white background on default logos (aws#9634)

This will ensure they remain legible even on a dark backrgound, as for
example where they are used as the icon image for NuGet packages within
a dark mode IDE window.

Fixes aws#9577


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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: RomainMuller