RICO is for people who either set accents in their city, want to have unique buildings that function in the vanilla game's RICO categories or, as the most extreme case, want to plop their whole city. In the last case, the whole growth and upgrade mechanics are taken out of the game.
Adding buildings to styles allows you to make uniform districts and have some control over what the game loads and what it doesn't load, but it's rather limited, as you can have only one style per district. Building themes are more versatile, as you can have several at once active per district. The mod will add active styles automatically as themes. Having several themes per district active allows for more control. For example, if you have lots of small footprint buildings in your theme (for a historical city for example) and a few larger footprint ones, the game will nearly always grow those larger buildings as long as it can. You can split the theme in two to let you grow some small footprint buildings first and activate the large footprint theme later to give a more diverse look. As always, you need a full chain of all levels from level 1 to whatever your max level is for all footprints in your styles/themes for them to grow. The mod has a cloning feature to fill missing spots, increase diversity of levels, or you just add vanilla buildings of the right size.
All this will increase the memory footprint of your game by quite a big chunk. Use the Loading Screen Mod to exclude unused buildings from loading and only load one instance of textures for all buildings that share them. You can also prevent all unused vanilla buildings from loading (as per the initial thread question), which is configurable per category (RICO, high or low density) or even down to individual buildings (exclude vanilla buildings you don't want to see in this city), see here:
Personally, I use a mix of building themes and RICO. For example, if I have a theme or mixed themes of Scandinavian residential row houses and only want one or two low density commercials in that district, I may just plop those if I don't have a fitting theme anyway.
