tbh, I'd say using units from the whole codex almost disqualifies you from calling your army "themed". By imposing limitations on yourself by not using the whole codex, surely you are gimping your army, not making it stronger? You can make up fluff for your marine chapter and paint new iconography on it, but if you use the whole codex, then basically its just ultramarines with a custom paint job. (Nowt wrong with that, mind you. My eldar fall firmly into this category.)
However, if I take 2 units of 10 wraithguard with warlocks, 3 wraithlords, a farseer and a wraithseer, am I spamming or am I just playing Iyanden according to the fluff?
spamming is an entirely different thing to themeing. I wouldnt even say 2 of a unit is spamming. 2 units of eldar guardians are spam? 2 units of dark reapers? 2 wraithlords? who decides which units are spammy and which units arent?
I think spamming is when you choose units which you deem to be very powerful against most opponents and max out on them. Not one or two. Max out.
Theming is when you think about the fluff and background of your army, and build an army which matches it as closely as you can, regardless of the playability of said build.
that's my two yen, anyway. Take with as little or as much salt as you like. We're all entitled to our opinions (except Pikey. He's barred