Below, is a 5 pixel thick solid purple line (hex code #441468) that runs 100% of the width of the column (any column):
Change the line thickness, enter the hex code for the color and percentage of column width — and you have the line that you want, where you want it.
Here's how. On the Elements tab, select More, then Custom HTML and place it where you would like your dividing line.
<div id="divider" style="background-color:#441468; height: 5px; width:100%;"></div>
and paste it into the block. NOTE: this code has been updated to work with Weebly's new setup (see comments, below, about my initial code that had stopped working).
If you want to use the divider again, you can repeat the process, copying and pasting your customized code into new Custom HTML blocks wherever you like.