To add on to @jwilkins.oboe’s remark about responsiveness, when you don’t have a lot of horizontal room to begin with you want to take advantage of all of it. When I narrow the browser all the way in, the blue box takes up only 55% of the horizontal space. Why limit yourself to 55% of the width when you barely have any width to begin with? At narrow widths the blue box should be taking up almost all of the width so that you have more room to display the content.
My recommendation, use a narrow-first (or mobile-first if you prefer that term) approach. Narrow your browser as far as it will go and style the page so it looks good at that narrow width. This is your base CSS. Then you can gradually widen the browser and add break point changes if needed for wider view ports. Personally, I don’t think you need any break points for a simple layout like this if you are smart about your CSS.