This help article will demonstrate a step by step tutorial how to customize the ControlDefault theme for RadCollapsiblePanel.
Export the built-in themes. in a specific folder by selecting File >> Export Built-in Themes.
Load a desired theme from the just exported files by selecting File >> Open Package
Navigate to CollapsiblePanelHeaderElment in
Controls Structureon the left side.
Modify the applied BackColors of the HeaderLineElement.
Save the theme by selecting File >> Save As.
Now, you can apply your custom theme to RadCollapsiblePanel by using the approach demonstrated in the following link: Using custom themes