[Cainehill]

Blood? Well, even on a non-blood nation, having a bit of blood on the pretender can be great for forging a couple of very nice items - blood thorn (only single handed weapon in the game with lifestealing), or blood stones.

Other than that, since I tend to play non-blood nations, I don't plan on using a lot of blood. However! There are magic sites that can easily make it worthwhile to go with blood. For instance - if you find the site that gives a blood-60 bonus (60% fewer blood slaves required for each summoning there), or a site that gives cheap powerful blood mages, it can be well worth cranking up a blood economy.

About the themes - most simply change the units and commanders the nation gets; some modify the gem income and required scales, and a very few (death themes mostly) heavily modify other things. But mostly, they change the units.

Some themes play very similarly to the base nation; others are wildly different. Conquerors of the Sea Marignon, for instance, is very different from standard Marignon, with very different magic strengths and with sailing instead of stealth.

Niefelheim theme for Jotunheim gives honking huge combat monsters, both commanders and troops, all very expensive and all sacred, great with a strong bless effect. On the other paw, Niefelheim loses the cheap researchers base Jotunheim has, but still plays fairly similarly to the base.