You can edit the design of buttons (and other design elements) by clicking on the Design button underneath the video title on the right hand side when you are editing your video. You will then see sections for configuring your video designs. Click "Button Styles" to edit the colors, rounding, and border of your buttons.

Additionally, you can set design templates at your Account or Group level that will automatically be inherited by videos so you don’t have to configure the same settings each time.