There are already some CSS classes declared, to make your designing a little easier. As you may have already noticed, many are already being used in the basic interface you get by default.

Class Description
center Positions the element in the horizontal center of the screen.
left Positions the element in the left side of the screen.
right Positions the element in the right side of the screen.
bottom Positions the element in the bottom of the screen.
middle Positions the element in the middle of the screen, vertically and horizontally.
align-left Aligns the content to the left.
align-center Aligns the content to the center.
align-right Aligns the content to the right.
vertical Distributes the content vertically.
horizontal Distributes the content horizontally.
dark It will change the letter color to a darker color. It must be placed in the quick-menu element for games with light backgrounds.
light It will change the letter color to a lighter color. Must be placed in the body for games with dark backgrounds.