Enhance dark mode map styles

[zacharlie]

On the whole, some serious consideration needs to accessibility enhancements, but in particular, dark mode seems to be a notable challenge.

The OpenLayers management and shortcodes in the Web UI are still rather preliminary, but with the migration to supporting standard hugo themes a bunch of artefacts crop up, especially in dark mode. Some of the artefacts are related to widget and component styles, such as layer name colours in the TOC, and others are related to map content, such as a background colour change.

It would be nice to get to the point where GIS practitioners can create optimized themes in a QGIS project, and have OSGS and QGIS Server handle rendering the correct one based on options specified in the UI, but for now just minimising the artefacts (e.g. having a background base colour for maps) will do.