Change BTT 'closed' Icon

I'm having difficulty finding the way to change the default icon used for BTT when it's closed/condensed. I'm using AQT and see it's been changed from the BTT logo to the AQT logo, so I'm guessing it's possible, but under which section/app can I modify this icon? (in the image below, I'd like to modify the icon 2nd from the right)

Thank you!

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