Instructions
Start a game of "RollerCoaster Tycoon" in Sandbox mode.
Select the area of your park that will fit in with your particular theme and click on the "Terrain" tool to alter it appropriately. For example, add lot of water for an aquatic theme; steep hills and mountains for an adventure theme.
Open the "Scenery" menu and hover the mouse over the icons at the top of the screen to browse through different themes. Select your theme. Place plenty of accessories around your themed area.
Open the "Rides" window and hover your mouse over the icons at the top of the screen to find out which themes they represent.
Click on the theme you want to use. The Rides window will display only rides which fit with that particular theme.
Select a ride. A transparent image of it will appear on the main game screen. Move this image to the location where you want to place the ride and left-click to build it.