Symptom
Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental.
Resolution
Yes, you can change it in Editor in the attributes panel on the right side. Select the lane first. This non-gradient option is called "flat design" which you will find in the section "More attributes".
If you have administrator rights in your workspace, you can change this setting globally. To do so, go to "Setup" -> "Define notations/attributes" select "BPMN" -> "Lane" and change tab to "Default formats". There you can configure the default "Solid color background" for all BPMN 2.0 processes. All customized processes won't be changed.
Keywords
KBA , BPI-SIG-PM-MOD-EDT , Graphical Editor for SAP Signavio Process Manager , How To