Add manual dark/light theme toggle

[M-J-Robbins]

Add a dark/light them toggle to override system settings if a user wants to.

Should have 3 options, light, dark and default system setting.

My initial thought is to store this as a URL parameter to keep things simple

Could also look at adding a reduced transparency, reduced motions, increased contrast, font-size etc.

[M-J-Robbins]

I've already quickly added CSS for this by adding a class to the html element of dark or light.

I've added a CSS transition to ease the colour change.