Transparent images in issue texts makes them unreadable

[HolgerJeromin]

If there are transparent images the black color shines through.
See these links:

Render vending=parcel_pickup;parcel_mail_in

or /gravitystorm/openstreetmap-carto/pull/3829

ref Transparent images in issue texts makes them unreadable · Issue #983 · StylishThemes/GitHub-Dark · GitHub