How I can customize with my own styles?

Switch settings to Advance and go to ‘Other‘ tab to fill “Custom CSS” field.

Example:

/* header */
#upprev_box h6 a
{
    color: red;
}
/* header link: mouse over */
#upprev_box h6 a:hover
{
    color: black;
}
/* headera link: visited */
#upprev_box h6 a:visited
{
    color: gray;
}
/* container for title, thumbnail and excerpt */
#upprev_box .upprev_excerpt
{
    font-size: 8px;
}
/* excerpt */
#upprev_box .upprev_excerpt p
{
    line-height: 1em;
}
/* previous post link */
#upprev_box .upprev_excerpt p a
{
    text-transform: uppercase;
}
/* previous post link: mouse over */
#upprev_box .upprev_excerpt p a:hover
{
    text-decoration: underline;
}
/* previous post link: visited */
#upprev_box .upprev_excerpt p a:visited
{
    color: gray;
}
/* thumbnail image */
#upprev_box .upprev_thumb
{
    padding: 1px;
    border: 1px dotted red;
}
/* close button */
#upprev_close
{
    opacity: .7;
}