Simpla is fixed width. There is a fluid width version named Simpla - Liquid.

Download Simpla at drupal.org/project/simpla. This version is 6.x-1.7 from 2009-May-18, a 60.83 KB download.

Validation as XHTML 1.0 Strict shows an error in the following line. The name and id attributes must have the same value.
<a name="top" id="navigation-top">

Wave did not detect accessibility errors.

W3C CSS Validator results for http://d-theme.com/simpla (CSS level 3)
Sorry! We found the following errors (11)

URI : http://d-theme.com/sites/all/themes/simpla/simpla.css?n
352 .node-unpublished div.unpublished, .comment-unpublished div.unpublished Property word-wrap doesn't exist : break-word
460 .comment-inner Value Error : background Too many values or values are not recognized : 1px solid lightGrey
878 fieldset Property _background doesn't exist : #ededed
879 fieldset Property _border-width doesn't exist : 0
880 fieldset Property _border-right doesn't exist : 2px solid #c0c0c0
881 fieldset Property _border-bottom doesn't exist : 2px solid #c0c0c0
887 fieldset fieldset Property _background doesn't exist : #f7f7f7
888 fieldset fieldset Property _border-width doesn't exist : 0
889 fieldset fieldset Property _border-right doesn't exist : 2px solid #ddd
890 fieldset fieldset Property _border-bottom doesn't exist : 2px solid #ddd
URI : http://d-theme.com/sites/all/themes/simpla/layout-fixed.css?n
288 #header, #content, #navbar, #sidebar-left, #sidebar-right, #footer, #closure-blocks Property word-wrap doesn't exist : break-word

The W3C CSS Validator found all the above errors and more for CSS level 2.1.
W3C CSS Validator results for http://d-theme.com/simpla (CSS level 2.1)
Sorry! We found the following errors (15)

URI : http://d-theme.com/sites/all/themes/simpla/simpla.css?n
352 .node-unpublished div.unpublished, .comment-unpublished div.unpublished Property word-wrap doesn't exist : break-word
460 .comment-inner Value Error : background Too many values or values are not recognized : 1px solid lightGrey
878 fieldset Property _background doesn't exist : #ededed
879 fieldset Property _border-width doesn't exist : 0
880 fieldset Property _border-right doesn't exist : 2px solid #c0c0c0
881 fieldset Property _border-bottom doesn't exist : 2px solid #c0c0c0
887 fieldset fieldset Property _background doesn't exist : #f7f7f7
888 fieldset fieldset Property _border-width doesn't exist : 0
889 fieldset fieldset Property _border-right doesn't exist : 2px solid #ddd
890 fieldset fieldset Property _border-bottom doesn't exist : 2px solid #ddd
902 Unknown pseudo-element or pseudo-class :not
911 Unknown pseudo-element or pseudo-class :not
URI : http://d-theme.com/sites/all/themes/simpla/layout-fixed.css?n
288 #header, #content, #navbar, #sidebar-left, #sidebar-right, #footer, #closure-blocks Property word-wrap doesn't exist : break-word
299 #page Property overflow-y doesn't exist in CSS level 2.1 but exists in [css3] : hidden
URI : http://d-theme.com/sites/all/themes/simpla/print.css?n
58 #page Property overflow-y doesn't exist in CSS level 2.1 but exists in [css3] : visible