[2020] District Styles vs. Building Themes?

I only have one thing to add to Termite's great explanation of the differences and that relates to spawning. If a style doesn't include something (a certain lot size or type of zoning), then the game will just fill that in with random buildings. As an example, if you have a style for low density residential, then you will just have random commercial grow in the district with that style applied.

Building Themes will ONLY allow buildings in a theme to spawn in a district. So in the same situation, where a theme only has low density residential buildings, you would have no commercial spawn at all. Ofc you can add multiple themes which makes it easy to deal with (I used to have several residential themes and then 1-2 themes with commercial, offices and industry), but it's something that's important to keep in mind in case you have nothing spawn in a district. :)

If you set no style, then it will spawn a mix of all of the buildings.