Divider Web Part with colored background that works in "full-width column" layout
To allow a little more customization to the look & feel, I suggest a very simple web part that would allow you to do a full width divider section where the user could pick the background color and the height. The web part would then go edge to edge in the column it is placed in and allow for the user to design visual breaks between sections. A nice to have would be the ability to add some styled text (font, size, color) aligned to the left, middle, or right side of the divider.
For the idea to really add a lot of design value, the web part would need to be allowed in the Full-Width Column section layout (right now only images and hero web parts are allowed there).
Jim Morrison commented
Even just an HR horizontal rule would be great.