The NF particle Effects seem to crash my game with an error about svd not being created, or it'll crash RCT3 on startup or loading with an error abou Direct3D.
THe (minor) problem with the neon lighting scenery is that all the lights are blue-grey. All of them are.
Should I try re-downloading both packs?

I came across the same "no svd for sid" myself and found out it is the way the file is unziped.
when you unzip the file, unzip it directly to the "Themed" folder. My Computer > Hard drive c > Program files > Atari > RCT3 > Style > Themed.Do not copy and paste or drag and drop or you will encounter the error. Try it and see if it works

And your problem with the lights turning out blue-grey, have you tried changing their color, the stock color is black which looks blue-grey