Find the instructions on how to use the SkinBuilder at the bottom of the page.
If you are still using DHTMLX 3.x, you can use the old version of the Skin Builder tool: Skin Builder 3.x
DHTMLX Skin Builder is a online tool that helps you to define any color schema for the interface of your application based on the DHTMLX components (both Professional and Standard editions).
You can choose some preset color scheme from the available list or create your own. When you choose a preset or change any color/font settings, the preview will be reloaded with the new settings automatically.
When you got a color scheme you like, click the Download button to get a zip file with all necessary files (.css and images).
In the "codebase" folder remove (or rename) the default/current dhtmlx.css file and the "imgs" folder. Then unzip the downloaded .zip archive in the "codebase" folder.
To define the skin in your application code, use the name of the base skin (SkyBlue, Web, Terrace) in the setSkin() method. You can also just omit setSkin() method as the DHTMLX library has a built-in skin detector.