Looking at the screenshot at http://atherys.com/pack, I can somewhat understand calling it "medieval", but I don't quite understand what you mean about it being "dark".
Personally, I use PureBDCraft (and patches for mods). I'd say it qualifies as "whimsical", and there are lots of patches for mods to give them matching textures (the search box at the top tends to be more effective than the list found at the "Patches" link), though it doesn't exactly keep the original style of Minecraft, and there are a couple things to be cautious of:
1. sometimes there will be more than one thread for a given mod, and some of those threads will only have downloads for older versions of Minecraft.
2. many of the downloads are through adf.ly links, but if you have AdBlock Plus, or stay careful to use the "skip ad" link that appears in the upper right corner after a few seconds and ignore any links that are part of ads, that won't be a big deal.
As far as keeping the original style of Minecraft, that's what the Faithful pack claims to do, and there is a link to matching packs for mods, though I don't see Metallurgy or Thermal Foundation listed (a forum or Google search might turn up some unofficial "faithful" packs for them, I haven't checked). I haven't tried this pack myself, since I prefer to use a small list of mods and 128x or higher textures.