Dark theme improvements

[xmo-odoo]

• Add switcher in the UI.

  Initially couldn't be arsed because there's no way to ask the browser to do it (yet?) so gave up, however thought about it during the weekend and it should not be difficult to define mixins for the light and dark themes, and then just use that in both the media rules and class-bound rules.

• Make the links brighten up on hover.

  Currently they inherit the darkening behaviour of light mode.

• Dropdown-menus are still light mode.

  Missed them when I did color centralisation. Used in the applications / backend menu, and for the staging results.

• Lighten text, background, and maybe increase color saturation a bit.

  The heathens still refuse to embrace the sweet promise of oblivious and find the color theme "gloomy" and "lacking contrast".

  Nota: looked like an opportunity to try using oklch (as it's apparently a perceptual cylindrical color space where hsl is non-perceptual) sadly

  • libsass is outdated / abandoned and doesn't have support for the channel function, which is required to perform LCH channel extraction / manipulation
  • native CSS does not provide a way to extract channels so there is no way to define -rgb values programmatically (that I can find)

• Darken borders of staging cells.

  They're still in their light mode, which creates very bright bars on the side of the cells whereas in light mode they're slightly darker separations over the background.