Utilities/Sphinx: Give the Qt Help output a color scheme

The `qthelp` builder renders with Sphinx's `nonav` theme, so the color
scheme that `cmake.css` picks up from the `default` theme in the html
build is never generated for it.  The workaround that inlines the
`@import` in `cmake.css` silently falls back to `basic.css`, which
carries layout rules only, leaving the `.qch` pages with no styling of
their own.

Supply one, generated from a single template and two palettes.  Qt
Creator serves a documentation set's own `offline-dark.css` in place of
its `offline.css` or `offline-simple.css` when the Qt Creator theme is
dark, so link the stylesheet under one of those names and ship a dark
counterpart next to it.  Qt Creator 20 and older substitute only for
`offline-simple.css`, and request `offline.css` instead when the theme
is light, so install the light palette under both names.

The html build is unaffected: the extra stylesheet is linked, and its
static directory is added, only under the `qthelp` tag.