There are different possibilities for adapting the look and feel - depending on what you want to do:
Overwrite the style definition in individual controls by
specifying the style
property. Offered for all
controls holding text information inside (label, button, field, etc.) and for
all container controls (areas, tables, rows, etc.).
Exchange the central style sheet file containing all style information for controls. Furthermore, specify your own style sheet: define a style sheet file for a page statically or switch between style sheets dynamically (e.g. user-dependent).
Create new controls by yourself and place them into the Application Designer design and runtime environment.
Option 1 is typically used if you like the default style provided by Application Designer - but you want to change it for some pages. For example, you want the text of the button to appear in red - instead of black for some buttons.
Option 2 is typically used if you have to adapt the style of the controls to some customer-specific style. For example, if you want to change the font "Verdana" that is used inside the Application Designer style, or if you want to introduce a new color scheme. Option 2 does not require any changes inside the page layout definitions - the style is completely separated from the layout. You do not have to regenerate your XML definitions at all.
Option 3 is used if you need new controls. There is an open API that allows you to add your own controls in a simple way.
Option 1 is discussed in Working with Controls (in the Layout Elements documentation). Option 3 is explained in the Custom Controls documentation. This part focuses on option 2 - exchanging the style sheet.