better solution for shared template and css files

Currently odk1 and and odk2 have duplicated css and template customizations.

There are some things (like the color scheme changes) that need to be odk1/odk2 specific, but I think we want to keep shared styles/customizations shared.

Start by inspecting the Sphinx documentation structure and the duplicated CSS and template customizations for odk1 and odk2. Identify which styles are shared and which must remain edition-specific, then confirm that both documentation variants build correctly with shared customizations and preserved color-scheme differences.

Written by the indexing model from the issue text.