As of this post, Quark is the default theme for Grav websites. It has a number of configurable options that you may want to check out.
To access these options, go to the Themes tab in your admin panel and click on Quark. The most likely options you will be interested in are the grid size and custom logo options.
The grid size determines the maximum width of the frame for your website and by default is set to Large. On a large monitor Extra Large might be preferable, as Large can create large swathes of empty space on either side of the blog. On the other hand, Extra Large may have too little space on either side of the blog for anyone using a normal size monitor. Try changing the settings to see what you like best.
The custom logo replaces the Grav logo at the top left. You could use a personal or organization logo, or even take a screenshot of your name in a nice font.