Specify max width for reflink preview in MDC theme

boards/static/css/md_centered/base_page.css

@@ -1,16 +1,16 b''
 @import url("../md/base_page.css");
 
-.thread, .post-form-w, .image-mode-tab, .tag_info, .bar-bg, .page_link {
+.thread, .post-form-w, .image-mode-tab, .tag_info, .bar-bg, .page_link, .post_preview {
     max-width: 1000px;                                                                
     margin-left: auto;                                                                
     margin-right: auto;                                                               
 }
 
 .thread {
     border-left: solid 2px #888;                                                      
     border-right: solid 2px #888;
 }
 
 .last-replies {
     margin-right: 3ex;
 }