Switching themes from custom.html

I realise this was posted both on GitHub and here, but just for others, the No value for t error occurs because Gadgetbridge expects any JSON object it receives on a line (eg {"a":42}) to be for it - and if it doesn't see a t field there it complains: https://www.espruino.com/Gadgetbridge#ba­ngle-android

To work around it instead of using Puck.eval to read the file directly, you just do Puck.eval([...]) which then returns what you requested as an array ([{"a":42}]) and then Gadgetbridge doesn't care