Instead of unpacking it like that you can just drag them to the desktop, works every time for me.

Actually, a lot of people use the technique that helped me. Right-click the folder, select "Extract Here".
Find the extracted version of the folder. Check it for "stylecommon" and "styleunique" OVL files.
Right-click the folder with those files. Select "cut" or "copy". Go to C:\Program Files\Steam\steamapps\common\Roller Coaster
Tycoon 3 Gold\Style\Themed. Or, if you didn't get RCT3 from Steam, go to C:\Program Files\Atari\RCT3\Style\Themed.
Right-click an empty space in the file, select "Paste". Test it out for yourself.

This usually works for me for some reason: after you extract from their .Zip, Just open each folder and then back out before dragging them into the style/themed folder. I dunno why but every time I do this, I never get an :SVD error