Add meta theme color

[beaufortfrancois]

This PR adds missing <meta name="theme-color" content="#141216"/> as suggested by Lighthouse.

Issue: #142

[dero]

This looks good if we want to just appease Lighthouse, approving. 👍

There are, however, some considerations around the dark mode. Right now the dark mode is applied when:

1. It's enabled in app settings.
2. It's enabled in the OS (trumps app settings).

While we could do something like this to change the theme-color when dark mode is enabled in the OS:

<meta name="theme-color" content="#fff"/>
<meta name="theme-color" content="#141216" media="(prefers-color-scheme: dark)" />

... we would then still need to dynamically change the former value when dark mode is enabled in the app using settings.

So it seems we might want to set the theme-color dynamically during app initialization based on the --background CSS property:

// Not tested, but something along these lines should work.
const metaThemeColorElement = document.querySelector('meta[name="theme-color"]');
const currentThemeColor = getComputedStyle(document.documentElement).getPropertyValue('--background').trim()

if (currentThemeColor) {
  metaThemeColorElement.setAttribute('content', currentThemeColor);
}

[derekherman]

It would make sense to fix it for both light and dark mode.

[beaufortfrancois]

Good catch indeed! I've handled light, dark and forced dark mode in latest commits. Please have a look.

[dero]

Looks great, thanks!