Despite the simplicity of the commit title, this was a pretty big
change. The styling used to just go off of the system's color scheme,
but that can't be overridden. Instead, I have made a variable that
determines whether dark theme is active and made a small panel with some
buttons to change the theme. I had to change a lot of code to achieve
this and lost a lot of hair (I metaphorically pulled it out) from
writing this code.
I also changed things from legacy mode to rune mode (Svelte 4 to 5)
while I was at it, that wasn't too big.
