You change how a control looks using the Styling Assistant, which can be found in its Properties tab under Control Style.
The Styling Assistant will show basic properties for that control. The available options will vary depending on the control's type.
Here, you can set a control's width and height, its background colour, border, padding and margin.
These properties will generate the appropriate CSS for you when the form is run.
Just like with CSS, child controls will inherit styling properties from its parent unless overwritten with its own settings.