Hi Marcin Kurowski,
Thank you for contacting Syncfusion support.
We have checked your query "It is possible to modify the Border width of FloatWindow in Office2007 theme of DockingManager". Currently, we have support only for Metro theme to customize the FloatWindow Border width in DockingManager. By using MetroBorderWidth property of DockingManager, we can customize the Border width of FloatWindow for Metro theme. We have prepared the sample for the same and you can download it from the below link.
But, we do not have this support in Office2007 or other themes of DockingManager. Could you please confirm us whether you are asking the same support to customize the Border width of FloatWindow in other themes of DockingManager like in Metro theme. This would help us to proceed further.