Ok this file is generated automatically from files:
--versionyui--/widget-base/assets/skins/sam/widget-base.css
--versionyui--/widget-stack/assets/skins/sam/widget-stack.css
--versionyui--/overlay/assets/skins/sam/overlay.css
--versionyui--/widget-modality/assets/skins/sam/widget-modality.css
--versionyui--/panel/assets/skins/sam/panel.css
At least in my case