Do you mean Thread titles or the text that your type in posts ? be more specific when asking a question.

when users are creating a post they may use the upper bar, where it is possible to select the fonts. Times is one of them.
If you want all the forum posts appear in Times font (dis-regardless of the selected by users font) go to CP->Design->Design management (CSS)->add this .posttdMessage {font-family: 'Times' !important;}->Save

12a3, you need to find .posttdMessage in your CSS and change it with the code what Natashko provided.

/*--------------------- Forum start ---------------------*/

.forumNamesBar {font-weight:bold;font-size:9pt;}
.forumContent { margin: 0px 40px 0px 40px;font-size:11px;clear: both;}
.forumContent td { color: #6d6d6d; font-size:12px;padding: 3px;}
.forumContent a { color: #1E90FF;}
.forumContent a:hover { text-decoration: none;}
.forumContent table td {border-collapse: collapse;font-size: 12px;}
.forumIcoTd,.forumThreadTd,.forumPostTd {padding:2px;background:#e8f2f6}
.gTable, .postTable { background:url('') repeat-x 0 -20px;;}
.gTable td { background: #ffffff; border: 1px solid #cccccc;}

.postTable td { border-collapse: collapse; border: 1px solid #efefef;}
.gTableSubTop {padding:2px;background:#e8f2f6;color:#000000;height:20px;font-size:10px;}
.gTable td.gTableTop { font: 14px/28px 'Tahoma'; color: #feffff; text-transform: uppercase; padding: 0px 10px; background:url('/topbar.jpg');}
.gTableTop a { color: #feffff;}
.gTableTop a:hover {}
.gTable td.gTableSubTop { background: #e8f2f6; color: #6d6d6d;}
.gTableSubTop a { background:url('');}
.gTableTop, .gTableSubTop, .forumNameTd, .forumThreadTd, .forumPosrTd, , .threadNametd, .threadPostTd, .threadAuthTd, .threadViewTd, .threadLastPostTd, td.threadsDetails, td.forumOnlineBar { padding: 3px 10px !important;}
#frM6 td, #frM14 td, #frM50 td, #frM56 td, #frM59 td {padding: 3px 10px !important;}

.postBottom table td {border: 0px;}
.postTable td {padding: 3px 10px !important;}
.postSeparator {display: none;}
td.posttdMessage {text-align: justify; font-size:12px !important;color: #000000;}
.posttdMessage {font-family: 'Times' !important;}
.threadPostTd, .threadViewTd, .threadAuthTd, .forumThreadTd, .forumPostTd, .postTdInfo {text-align: center !important;}
.threadType{background: #cccccc; color: #444;}
a.TopSortLink {}
a.TopSortLink:hover {}
a.postUser {font-weight: bold;}
a.forum, a.threadLink {font-weight:bold ;font: 12px 'Arial';}
.forumDescr, .threadDescr {font: 12px 'Arial';}
td.postTdTop {background: #cccccc; color: #444;}
td.postTdTop a {color: #444;}
td.forumIcoTd, td.forumThreadTd, td.forumPostTd, td.gTableRight, td.threadIcoTd, td.threadPostTd, td.threadViewTd { background: #e8f2f6;}
td.pollQuestion, .pollResults td, td.pollResults, td.pollTotal, td.pollAnswer, td.pollButtons, .smiles td{ border: 0px;}
td.pagesInfo, td.switch, td.switchActive {background: #ffffff; border: 1px solid #dcdbe0; padding-left: 5px !important; padding-right: 5px !important;}
td.switch, td.switchActive { background: #6d6d6d; color: #ffffff;}
td.switch a {color: #ffffff;}
.switchActive {font-weight: bold;}
.smiles td {padding: 2px 5px !important;}
.codeMessage {background:#F0F8FF;-moz-border-radius: 10px; /* Firefox */
-webkit-border-radius: 10px; /* Safari and Google Chrome */
-khtml-border-radius: 5px; /* Linux browsers */
;font-size:12px;}
.quoteMessage {background:#F0F8FF;-moz-border-radius: 10px; /* Firefox */
-webkit-border-radius: 10px; /* Safari and Google Chrome */
-khtml-border-radius: 5px; /* Linux browsers */
;font-size:12px;}
.bbCodeBlock {padding-left: 30px !important;}
.posttdMessage img {
max-width:565px;
width:expression((this.width>450) ? "450px" : true);
}
.posttdMessage img {
max-height:500px;
height:expression((this.height>500) ? "500px" : true);
}

a:hover{
text-decoration: underline;
color : #0000FF;
font-weight:bold;
/*background-image:url(http://ghost2s.net/softvn_v2/img/line_rainbow.gif); */
}

/*icon bai viet*/
.thDescr,
.threadDescr {display:none}
#frM30 img {opacity:.33; -moz-opacity: 0.33; filter: alpha(opacity=33);}
#frM30 img:hover {opacity:.5; -moz-opacity: 0.5; filter: alpha(opacity=50);}
#frM30 img.selectedImg {opacity:1; ; -moz-opacity: 1; filter: alpha(opacity=100);}
/*end icon bai viet*/
/*--------------------- Forum end ---------------------*/

12a3, we have checked your website and it worked just fine. All the posts on your forum are in Times font now. Wasn't that what you wanted?? Be specific.