If you want something that sounds very close to Theories in style, then Darkness in a different light is your best bet
But if you want to open it up a little, I would recommend my two favorite FW albums
1. Pleasant shade of grey
2. Perfect symmetry
Both are Ray Alder.
Parallels is also a very well liked album. Honestly, there's no Alder era album I don't like, so you can almost pick anything and get something out of it.
I agree with all of this.
I would say that Darkness is most similar to Theories, but maybe a bit heavier/darker (I prefer Darkness by a pretty wide margin).
Disconnected and FWX are pretty similar. Both are brooding and atmospheric. For me, these are at the bottom of the list of Ray Alder albums.
Parallels and Inside Out are fairly similar. FW's most commercial sounding albums, but both have a good sprinkling of prog, and both are excellent.
No Exit is very Raw - not dissimilar to Warning/Mindcrime era Queensryche. This album contains the band's only side-long epic, The Ivory Gate of Dreams, which is structurally similar to Rush's 2112, but heavier and a bit more proggy.
Perfect Symmetry is next level above No Exit. They got a new, more technically skilled drummer, and the album is much more proggy. Probably my second favorite after APSOG.
A Pleasant Shade of Gray is probably most dissimilar to the rest of the catalog. It's technically one 54 minute song broken into 12 tracks, The majority of opinions I've heard absolutely love it, with a small, but not insignificant, minority who don't really care for it. Not a lot of middle of the road opinions. It took me a little while to get into it, but I could listen to it on a loop for weeks at a time.
I almost forgot the three John Arch albums (Arch was the singer on the first three, mid-80s albums, Night on Brocken, The Spectre Within and Awaken the Guardian). Arch is a very different type of singer (and was the primary lyricist when he was in the band and was a very different lyricist from Jim Matheos), and there's a decent chunk of the fan base that doesn't really care for Arch's singing. Personally, I think Alder is vastly superior in every way, but I don't dislike the Arch albums.
I think Guardian is the favorite Arch era album of a majority of fans. It's the last Arch album and, not surprisingly, probably the most mature. Brocken is the debut. It's VERY raw and kind of a poor man's Iron Maiden clone. Damnation is the only song that really gets any mention. Spectre is, for me, hit or miss. Traveler in Time and The Apparition are two of my three favorite Arch era songs.