The CSS validates at level 2.1 and has just one error at level 3. Perhaps that one line could change. (http://d-theme.com/blix)
URI : http://d-theme.com/sites/all/themes/blix/style.css?s
519 #wp-calendar Value Error : display inline-table is not a display value : inline-table

The theme is marked XHTML 1.0 Transitional and validates as XHTML 1.0 Strict. Perhaps the doctype could also move up.