Hi Stefan,
Thanks for your interest in Syncfusion products.
SymbolPalette supports Background with any kind of Brush, We have represented Custom Palette in FeaturesDemo sample from Dashboard, by clicking on MenuItem named ‘CustomPalette’, SymbolPalette’s Background will be set to DrawingBrush. However, for your continence we have created a sample with LinearGradientBrush, please refer the sample from the following link.
Sample Link:
http://files.syncfusion.com/Support/Diagram.WPF/Samples/F80661/SymbolPaletteBackground.zip For more details about customizing the look and feel of SymbolPalette, please refer online Documentation
http://help.syncfusion.com/ug_83/User%20Interface/WPF/Diagram/ and navigate to Concepts and Features -> SymbolPalette -> Customize the SymbolPalette.
Please let us know if you have any concerns.
Regards,
Jegan