Gradient theme backgrounds

Loomio sites can now use an optional gradient for the application background in light mode, dark mode, or both. When a gradient is not configured, the application continues to use the existing solid theme background.

Sites can also override the text colors used on theme backgrounds and surfaces, independently for light and dark modes.

Anchor text uses the primary color for its mode unless a separate anchor color is configured.

Application and thread navigation drawers can be colored independently. Without explicit drawer colors, the application drawer uses the surface color and the thread drawer uses the background color.

When a background gradient is configured, the application bar uses the same fixed gradient after the page is scrolled. Without a gradient, it can use a separate scrolled color and falls back to the regular application bar color.