The quickest way is to check the “Make Backdrop Track” box in the Backdrop tab of your Text control window. From there, fiddle around with the controls or go to the newly created backdrop track nested inside your text track on the timeline and mess around with the associated Shape/Fill/Border/Shadow controls.
For more control, you could instead create individual color and/or spline tracks and adjust them to your tastes and needs. There are several pre-made text backdrops in the Keyframe Library, assuming you’ve already rendered out the thumbnails.
HTH