Switching themes from custom.html

Is there a way to autorun the app after installing from the app loader?

Not at the moment, no. If it's the only clock installed it'll be used, but while there are other clocks they will likely be used first.

However you could:

• use an interface.html which doesn't install an app but instead communicates direct with the Bangle - in that case it could just send a command to write the file and even load your app.
  
• if your app literally only sets the theme (and there's no actual app part) you could make it a 'RAM' app like assistedgps or fwupdate - it's kind of the same as interface.html but there's nothing to install - you just click the button and again you can communicate with the bangle