In the Apperance tab, you can choose the color for the player buttons. It also lets you choose a thumbnail.
From the left side panel simply choose a color using the color picker. You can also manually enter a hexadecimal value of the color.
Thumbnails is what viewers first see when interacting with your videos. Uploading your own image thumbnail is a great way to show viewers what your video is all about.
Still on the Appearance tab simply click on the "upload a new thumbnail image"
To delete the thumbnail, click on the trash icon