Text Box in Highend Presentation Slide

I find myself darkening the slide (i.e. image) to help make the title and subtitle text legible. Obviously I can change the text color to one that makes the text legible, but often the color isn’t aesthetically pleasing.

I would like to use the original photo and create a darkened text box. How and where would I add the functionality to create a text box around the text in a presentation slide?

Thank you for developing the Highend Theme. I’ve enjoyed learning its functionality.