CM says not to use PNGs, then uses PNGs in their template builder (?)

Campaign Monitor says not to use PNGs here:
https://www.campaignmonitor.com/resources/will-it-work/guidelines/

CampaignMonitor uses PNGs in the template builder, for example:
https://img.createsend1.com/img/templatebuilder/forward-glyph.png

Which is it?

And more importantly, where can I find a black version of that forward-glyph? :)

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