The module is inside a div of class ulti_counter. So to change the style, you'll have to add the style of this div in the css-file of your template.
e.g.:
div.ulti_counter {
color: #FF0000;
font-size: 20px;
}
If you put this in your css file, the text in the Ulti Counter module will be red and the size of the font will be 20px.