It is an online pedagogical best practice to break up large pages of text into more easily digested chunks, and to clearly delineate these chunks with white spaces, text boxes, line-separators and other tools. The Rich Content Editor in Canvas does not provide for this. However, the simple line of code below, can make this happen by placing a horizontal border line on a Canvas page.

<p style="border-top: 1px solid black;"> </p>

You will note that I have bracketed the code example above with a 4px (pixel) blue line above the code snippet, and a 1px (pixel) black line below the code.

Anatomy of the Code Snippet

Steps to Create a Horizontal Line on a Canvas Page

Select and copy the line of code above,
Navigate to an existing Canvas page and go to Edit mode,
Scroll down to the point where you would like to place your line, insert your cursor after the last line of text before your planned line, and then hit enter a couple times,
Switch to the HTML Editor,
Scroll down until you see the desired line of text followed by the two "<p> </p>" that were created by hitting enter twice,
Place your cursor after the first "<p> </p>" code, and hit enter,
Paste the copied code snippet there, then
Switch back to the Rich Content Editor to view the results.
To modify the code to make the line thicker or a different color:

Raise the number of pixels (yellow highlighted text above) to make the line thicker. I put some examples below.
Replace the blue highlighted text in the code snippet above with the color you want to use. I have tested this code with the following colors, and can verify they work.

Blue,
Red,
Green,
Yellow,
Pink, and
Purple

Troubleshooting

If you find that a line of text is crowding your horizontal line, either before or after the line, then go back into the HTML Editor, scroll down to the line of code you added and place a "<p> </p>" (without the quotation marks) before or after it as appropriate.

Examples

Black line 1px thick

Blue line 4px thick

Red line 6px thick

Green line 8px thick

Purple line 12px thick

Yellow line 16px thick

Orange line 20px thick

Pink line 24px thick