When I begin working on dark mode next week, I'll differentiate the tip button from the regular upvote button so that it's more obvious.
I'm going to add the following to the faq:
If you press the upvote button a second time, a dialog pops up to tip. Optionally you can set this amount as your default tip to avoid the dialog on subsequent tips.
On the settings page accessible from the dropdown menu. If you only want to temporarily vary your default tip, you can press and hold the upvote button to present the tipping dialog.