Implement Dark Theme and Theme Switcher

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!