@SrMorte:
1. Open file /templates/rt_zephyr_j15/css/extended.css and remove lines:
/* JCOMMENT */
#jc .rbox_tr, #jc .rbox_tl, #jc .rbox_t, #jc .rbox_br, #jc .rbox_bl, #jc .rbox_b, #jc .rbox_m {background: none;border: 0;}
#comments-form label, #comments-report-form label {padding-left: 15px;}
#jc .rbox div {line-height: 140%;}
#jc .rbox {border: 0;}
#jc .even .rbox {background: url(../images/overlays/dark/box1-bg.png) repeat;}
.body-overlay-light #jc .even .rbox {background: url(../images/overlays/light/box1-bg.png) repeat;}
#jc .odd .rbox {background: url(../images/overlays/dark/box2-bg.png) repeat;}
.body-overlay-light #jc .odd .rbox {background: url(../images/overlays/light/box2-bg.png) repeat;}
#jc #comments .quote {background: none;}
#comments-form p {margin-left: 0 !important;}
#comments .comments-list {margin-left: 0 !important;}
#comments-list-footer {margin: 0 0 10px 0 !important;}
#comments-footer, #comments .toolbar {border: 0;}
/* Quote Block */
#comments blockquote, #comments .hidden {background: url(../images/overlays/dark/box1-bg.png) repeat;border: 0;padding: 15px;color: inherit !important;}
.body-overlay-light #comments blockquote, .body-overlay-light #comments .hidden {background: url(../images/overlays/light/box1-bg.png) repeat;}
/* Input Form */
#comments-form input, #comments-report-form input, #comments-form textarea, #comments-form input.selected, #comments-form textarea.selected {background: url(../images/overlays/dark/box1-bg.png) transparent repeat !important;border: 0;}
.body-overlay-light #comments-form input, .body-overlay-light #comments-report-form input, .body-overlay-light #comments-form textarea, .body-overlay-light #comments-form input.selected, .body-overlay-light #comments-form textarea.selected {background: url(../images/overlays/light/box1-bg.png) transparent repeat !important;}
/* Form Button */
#comments-form p, #comments-form #comments-form-buttons, #comments-form #comments-form-error, #comments-form #comments-form-captcha-holder {margin-left: 0 !important;}
#comments-form .btn, #comments-report-form .btn {background: url(../images/overlays/dark/readon-r.png) no-repeat 100% 0;margin: 0 0 0 4px;}
.body-overlay-light #comments-form .btn, .body-overlay-light #comments-report-form .btn {background: url(../images/overlays/light/readon-r.png) no-repeat 100% 0;}
#comments-form .btn:hover, #comments-report-form .btn:hover {background-position: 100% -27px !important;}
#comments-form .btn div, #comments-report-form .btn div {background: url(../images/overlays/dark/readon-l.png) no-repeat 0 0;margin: 0 0 0 -4px;}
.body-overlay-light #comments-form .btn div, .body-overlay-light #comments-report-form .btn div {background: url(../images/overlays/light/readon-l.png) no-repeat 0 0;}
#comments-form .btn div:hover, #comments-report-form .btn div:hover {background-position: 0 -27px !important;}
#comments-form .btn a, #comments-report-form .btn a, #comments-form .btn a:hover, #comments-report-form .btn a:hover {font-weight: normal;color: inherit;}
/* Fonts */
#jc h4 {border: 0;color: inherit !important;}
#comments .comment-author, #comments .author-homepage {color: inherit !important;}
#comments a {color: inherit;}
2. Open file /templates/rt_zephyr_j15/features/styledeclaration.php and remove lines:
$css .= '#comments-form input, #comments-report-form input, #comments-form textarea, #comments-form input.selected, #comments-form textarea.selected {color: '.$gantry->get('body-text').';}'."\n";
After this JComments template's styles will not overrided by Joomla's template.