How can I change the thickness of my tag

For consistency remove any borders and use the height for the <hr> thickness. Adding a background color will style your <hr> with the height and color specified.

In your stylesheet:

hr {
    border: none;
    height: 1px;
    /* Set the hr color */
    color: #333; /* old IE */
    background-color: #333; /* Modern Browsers */
}

Or inline as you have it:

<hr style="height:1px;border:none;color:#333;background-color:#333;" />

Longer explanation here