What functionality is missing? What is unsatisfying with the current situation?

Almost every genre can be covered from the stock Themes in World Anvil except the category of Espionage/Military type settings and/or campaigns. For those of us who can only do basic CSS, the standard themes work great for their particular genres or 'themes' but none of them work very well for a more contemporary spy or military type project. The science fiction themes are too 'spacey', cyberpunk can be altered but changing the colors throws the whole theme off, and none of the fantasy pages do a very good job with their choices of fonts, background colors and presentation formats.

Game Types Contemplated-TWILIGHT 2000 (and variants), NIGHTS BLACK AGENTS, SPYCRAFT, COVERT ACTION, CALL OF CTHULHU-DELTA GREEN etc. In addition, such themes could be used to publish and communicate wargame results/information amongst those groups in a thematically appropriate format with minimal code work.  

How does this feature request address the current situation?

Adding one or two new Themes that convey a more modern, realistic but still 'espionage or military' feel to a project. It would be best to be approached with two separate themes but one could be utilized for both concepts. If two were contemplated:
ESPIONAGE | TECHNOTHRILLER-This would be a Clancey-esque Theme that appeared as a high-end government network with appearance similar to a 'movie-version' of a modern spy thriller's agency database.
MILITARY | UTILITARIAN-This would be a more subdued but straight-forward visualization to a project. Block fonts, templates adjusted to appear as military reports (subdued colors, black font).  

What are other uses for this feature request?

As with any other Theme, it can be used a stepping stone for a number of concepts. The existence of these themes would support those of who (despite being Grandmaster accounts) couldn't CSS ourselves out of a paper-bag. Those individuals with greater expertise would be able to launchpad more ambitious projects in these genres.