Implement Dark Theme and Theme Switcher

[cactysman]

These changes are a take on implementing a dark theme (and a theme switcher) for #205.

• since the splash screen is not a Vue component, it has some primitive @media(prefers-color-scheme: dark) styling
• the TheThemeChanger component has been implemented similar to how TheViewChanger was implemented
• the colors and the Ace editor's theme now depend on the system's color preference that can be overridden (but not persisted) with the TheThemeChanger component
• the colors for the dark theme are mostly derived from the light theme (sometimes just switching front and back colors) or have adjusted lightness

Note

The way the color variable management looks now might actually be a little sloppy and since I'm not a designer, the dark colors might not be ideal at all, but it's a starting point!

[cactysman]

Also I just blindly picked the "Twilight" theme (view themes here) for the Ace editor in the dark theme, that might require some more decision-making to find something that looks nice and fits in with the rest of the color scheme (note that the color for highlighted code has been and still is custom, though).