How to add a full-width separator?

To add a full-width separator, you need to go to Edit mode of your dashboard. Locate the right sidebar and select the Add Text Widget.
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Then follow these steps:

2. Add a description of your widget, or skip this step and click next;

3. Type in the Widget title, select the size 4x1, and choose Auto Height Card as the Format. Click on Submit at the end, et voila!

You now have a full-width separator!