Fix scrollHeader()
Fix scrollHeader()

The header did not react to scrolling since it changed from document to body
dimayakovlev/grav-theme-antimatter,dedude/grav-theme-antimatter,dimayakovlev/grav-theme-antimatter,getgrav/grav-theme-antimatter,franzyke/senhri-website-theme,dedude/grav-theme-antimatter,dimayakovlev/grav-theme-antimatter,getgrav/grav-theme-antimatter,getgrav/grav-theme-antimatter,franzyke/senhri-website-theme,dimayakovlev/grav-theme-antimatter,franzyke/senhri-website-theme,dedude/grav-theme-antimatter,franzyke/senhri-website-theme,dedude/grav-theme-antimatter,getgrav/grav-theme-antimatter