oops, didn't noticed that. Sorry.
i found another way:
this is coming from Arch Wiki:The background is black
This is most likely caused by not running a composite manager, like Xcompmgr or Cairo Compmgr. Cairo-dock uses the transparency feature of the composite manager to display the dock, and without it the dock will be displayed with a black background. If you are using a desktop environment, simply enable the composite manager or desktop effects in the settings.
An alternative solution that does not require a composite manager is to enable fake transparency in cairo-dock. To do this, right-click the dock and go to Cairo-Dock > Configure > Advanced Mode > System > Composition. Then enable both Emulate composition with fake transparency and Make the config panel transparent.