try this below see if it helps ipb_styles.css in the Look & Feel and look for .topic_buttons li.important a, .topic_buttons li.important span, .ipsButton .important,
.topic_buttons li a, .topic_buttons li span, .ipsButton {
background: #212121 url({style_images_url}/topic_button.png ) repeat-x top;
border: 1px solid #212121;
border-width: 1px 1px 0 1px;
-moz-border-radius: 3px;
-webkit-border-radius: 3px;
border-radius: 3px;
-moz-box-shadow: inset 0 1px 0 0 #5c5c5c, 0px 2px 3px rgba(0,0,0,0.2);
-webkit-box-shadow: inset 0 1px 0 0 #5c5c5c, 0px 2px 3px rgba(0,0,0,0.2);
box-shadow: inset 0 1px 0 0 #5c5c5c, 0px 2px 3px rgba(0,0,0,0.2);
color: #fff;
text-shadow: 0 -1px 0 #191919;
font: 300 12px/1.3 Helvetica, Arial, sans-serif;
line-height: 30px;
height: 30px;
padding: 0 10px;
text-align: center;
min-width: 125px;
display: inline-block;
cursor: pointer;next change the background: #212121 to whatever color u like #note the color background color may differ cause of the theme so if its not #212121 dont panic just change whatever color is used in the background, the above is what i got from the default theme