help a raw newbie choose a theme

I’ve been searching and previewing themes all day, with no success. I know exactly what I want my website to look like: in layout terms, I love https://www.charitywater.org/ as a model (my content will be much different and won’t look like a copycat). How close to that site does my theme need to start out? What are the limiting attributes that I have to have on the theme I choose? Can widgets and plugins make a large number of themes into something like that?