Use whatever engine you're comfortable with! 

Whether that's Twine, Unity, GameMaker, Renpy, RPG Maker, whichever you can make something in.

If you don't like any engine, why not make yourself a pastel board game? 

Remember to keep things simple, come up with a basic action or short story, you can always add more near the end of the jam. 

If you want to jump right into things, or don't want to spend a lot of time thinking of your own palette, there's plenty here: http://colorpalettes.net/category/pastel-color/

The best way to depict a pastel theme is to use primarily light colours, examples of pastel are in these games:

https://ebihime.itch.io/emmeline

https://nomnomnami.itch.io/lonely-wolf-treat

https://fromsmiling.itch.io/osteotic-bypass

https://sofdelux.itch.io/mermaid-splash

https://ninjadodo.itch.io/guppy

https://candle.itch.io/pastel-paradise

https://alittleredpanda.itch.io/pastel-dancer

https://boymakesnoise.itch.io/pastel