Switch to Gmail's old look
You may have noticed Gmail has introduced a new visual style, allowing more room for messages and smoothing lines and edges.
You may like it, but if not, there’s a way to temporarily switch back to the old look. To do so, click on the “gear” icon in the top right-hand corner and then click on “revert to the old look temporarily”, and then follow the prompts.
Keep in mind this is only a temporary solution and you’ll eventually have to switch, but if you can’t stand the new look just yet, this is a good solution.