User can customize their own channel, which consist of some parts. There is a background image, built-in color and opacity selector and layout options. All these can be tweaked to make your channel look like you want.
Youtube offers right from the get-go several themes with different color schemes. Those theme are very basic and don´t offer very much eye candy. For a Youtuber to be a successful video and channel producer he or she need to be aware that the look of the channel greatly effects how people feel about that channel.
A Theme is a lot more when you start to scratch the surface, post comments and thoughts about backgrounds, colors, layouts and themes if you want to add to the discussion. Gonna post more about the design aspect of it soon, so keep your eyes open. Be well.