Create a custom class, e.g. .custom-btn. Note that to override jQM styles without using !important, CSS hierarchy should be respected. .ui-btn.custom-class or .ui-input-btn.custom-class.
.ui-input-btn.custom-btn {
border:1px solid red;
text-decoration:none;
font-family:helvetica;
color:red;
background:url(img.png) repeat-x;
}
Add a data-wrapper-class to input. The custom class will be added to input wrapping div.
<input type="button" data-wrapper-class="custom-btn">
Input button is wrapped by a DIV with class ui-btn. You need to select that div and the input[type="submit"]. Using !important is essential to override Jquery Mobile styles.
div.ui-btn, input[type="submit"] {
border:1px solid red !important;
text-decoration:none !important;
font-family:helvetica !important;
color:red !important;
background:url(../images/btn_hover.png) repeat-x !important;
}