~snip code~
How would I use this?
(total n00b here) Don't spend too much time on it though, it looks like bitmover's solution works just fine already.
What is the logic of detecting low/high post numbers?
There's some overlap, but
51904009 is the first archive post with correct CSS.
One post older lacks the proper layout. My earliest archived posts were a bit messy and I did miss some of them due to down time.
Post 51904009 could also be found as part of "oldposts", this version was scraped a year later (after editing).
I want to practice my skills as well.
You're doing just fine

Done. Just removed all unused CSS. There is now a single file with 14kb, no more css file.
Great!
A small correction:
Remove the last 5 digits from the msgID to get the directory name (if there are less than 5 6 digits, use 0):
Very sharp! I made that mistake a couple more times. I changed it to "5 or less".
I didn't understand. Your
low post number (25000030) is
higher than the
high post number (5310200). Is it the opposite?
I missed a 5, the high number is 55310200.
With this clarified: is it possible to combine "msgID" and "Old Posts" into one field, with post 51904009 being the first one to go to "msgID" and anything lower than that going to "Old Posts"? That would be much better than having to explain the difference.
Please feel free to change text/instructions/title etc
I will

I haven't added "oldposts" to the member/topic-directories yet. I need some time to script it, it would indeed be better to have all posts archived in all categories.