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There are several ways to do that:
In Titler, just first create a Filled Shape (Rectangular Shape), and drag it to fill the entire Frame, then your Text. Note: no need to do a Stroke for that Shape.
Or, create a Color Matte (from the New Icon), and place it on a Video Track below your Title.
Or, create an Image in Photoshop/PSElements (I would just use one of the Video Image Presets, that matches your Project's Frame Size), and choose a Background of the desired color, then add the Text with the Text Tool. Save_As a PSD, or PNG, to be Imported into the PrE Project.