Are you using any external css files on the page
It does look as css conflict, be sure that you are not using css styles which change styling of all elements on the page ( adding padding to all div elements may cause similar effect )
Where did you even find the files needed for the Terrace skin in the latest 4.0 download? I know it requires a dhtmlxscheduler_dhx_terrace.js but i could not find it in the default download from the site? I was able to find the images and the css file though.