[Support request] How I can create this style for my widgets titles?

Hi there,

you can add some CSS to target the H2 in the single post and in the sidebar :

.single-post h2, .sidebar h2 {
    padding: 5px;
    background-color: #f00;
    color: #fff;
    text-shadow: 1px 1px 2px rgba(0,0,0,.3);
}

You can include the #reply-title selector in your CSS:

.single-post h2, 
.sidebar h2,
#reply-title {
    padding: 5px;
    background-color: #f00;
    color: #fff;
    text-shadow: 1px 1px 2px rgba(0,0,0,.3);
}