DevExpress offers you multiple ways to customize the appearance of an ASPxTreeList.
Cascading Style Sheet (CSS) Classes
Use cascading style sheet (CSS) classes. CSS styles define how elements are rendered, and their position on the page. This provides you with centralized control over the appearance of several controls, or the entire web site. The ASPxTreeList ships with predefined presets themes. You can also create your own themes.
The ASPxTreeList provides multiple styles, so you can customize the appearance of individual elements, and manage the tree list's overall appearance. These styles can be accessed via the ASPxTreeList.Styles property. The styles used to paint the pager can be accessed via the ASPxTreeList.StylesPager property.