Considering the world is nearly destroyed and the remnants of humanity are either gritty raiders or ridiculous idealists, and despite all that, you're tasked to still help out your fellow man... Would it be appropriate to say one of Fallout's themes is hope?

tl;dr Mankind screwed up a lot, but with perseverance, we'll be alright.

In actuality? Hope is probably an underlying theme.

In FO1, you save Vault 13 and defeat the Master from taking over the West Coast. In FO2, You Defeat the Enclave, thereby saving practically everyone on earth from a horrible death, and in FO3, you defeat the Enclave a second time, and guarentee fresh water to everyone in the DC wasteland.

Pretty much, no matter how dark the coming storm may be, there's always gonna be some sunshine when it's gone.

Or maybe in a world of evil and decay some good can still come of it.

without good there can be no evil!

Indeed. The idea that after the worst, we can bounce back.

I know seeing VC for the first time had an effect on me... Civilisation, Real Civilisation, even if it has a different moral code.

Given Fallout's gritty exterior, it's kind of nice to see there's more to it if you really look at it.

Unless I'm reading a little too deeply.

I don't think I would say that Fallout is particularly much about hope, or at least any more than any other RPG where you save things (which would be almost every single one created probably).

It's not a particularly cheerful outlook on things when you think about it. In Fallout 1, sure you can end up saving your Vault. But it turns out the people you were saving aren't on your side after all, you're not welcomed after your long and tough journey. You're exiled from your home.
In Fallout 2, sure things have gone more civilized, but the world is hardly a good place.

I mean, hope will obviously be a part of a world such as Fallout's, or else there would not be any interesting contrasts. But the general outlook is too bleak, the spotlight is much more on the bad sides of humanity and how they can screw everything up and the irony of it all (which is visually represented through the cheery 50s style contrasted against the harsh wasteland). So I would personally not call it a main theme of Fallout, even though it obviously is present in the gameworld.

Hope thrives in the NCR, not so much D.C. The NCR had 127 years of rebuilding and cooperation versus the Capital Wasteland, which was just as FUBAR as if the bombs were dropped a week ago, even with the BOS trying to protect people. NCR is more like A Canticle for Liebowitz whereas C.W. is more like the shiftless hellscape of A Boy and his Dog.

Actually there was really nobody in the original games you could describe as being a "ridiculous idealist"...

I think the main theme in Fallout in General Sheperds word is that "Uniforms are relics, battlefields are history; war rages on all around us and it's our duty to either stop it or start it.

The master was an idealist... Unity and peace through (Forced) evolution.

The Enclaves in Fallout 2 fit this bill really well.

I think hope is a good theme but there are some underlying themes. "You only get what you give" and "if you want something done right do it yourself". Things were not going so good till the lone wanderers in the games Helped the people, by helping them the wander helped him/her self. The people followed his/her example and things became alot better in california area, But hope is good over all sums things up nicely.

When playing Fallout 3, I always went with the decisions that I thought would benefit the Human Race as a whole in the long term.