Most of the layout is determined by your theme, so check your theme options. I try to stick by the philosophy that themes control the appearance, and plugins add functionality, as much as possible. So, the amount of CSS my plugin adds is the bare minimum to make things look decent, but without infringing on things like font size and styles, which should be left to the theme (or Custom CSS).
Other than that, you can use the Customizer Extension here:
https://stephensherrardplugins.com/plugins/sign-up-sheets-customizer/
which allows you to change things like font size, color, cell padding (for the tables), and some other things without having to know CSS.
Everything has its own class or is in a div container with a class, so it’s extremely easy to target specific elements with CSS and modify them however you want.
If that’s not something you are good at, or want to take the time to learn (it’s very easy), then you can use my own Customizer extension for a bit more control over the appearance, or you can use something like the Microthemer plugin to target and style things the way you want (although that can be confusing also if you are new to CSS). https://ww.wp.xz.cn/plugins/microthemer/
