:art: assets: Comment and organize CSS stylesheet

[jwflory]

This commit is a complete reorganization of the custom CSS stylesheet used in the UNICEF Inventory theme. Most CSS rules are now grouped together in like sections with other rules. It is easier to read the file and also understand which rules are associated to which part of the theme. This is an effort to reduce technical debt and improve ease of maintenance for future maintainers.

Note: This is a major change to the CSS stylesheet. Any pending or future Pull Requests made after this commit is merged will likely have a merge conflict and likely will need to be merged by hand.