Paprastas būdas custom stylinti input[type=”text”] mygtuką

<form>
    <label>Upload Picture
        <input type="file" name="picture" />
    </label>
</form>  

label {
    position: relative;
    background: green;
    padding: 15px 30px;
    color:#fff;
    border-radius: 25px;
    display: inline-block;
    cursor: pointer;
}
label input[type=file] {     
    outline: none;           
    position: absolute;
    left: 0;
    height: 0px;
    width: 0px;
    line-height: 0px;
}

Snipetas:
http://jsfiddle.net/3bXGw/1/