Why would the default stylesheet here show recent posts in the same color but on my site displays them in alternate colors?

Like I said you may need to edit a couple of properties.
But I believe the alternating colors was the original default setting. Dark light dark light

Maybe I am going crazy, or just do not understand the logic, but would it not be more logical if the file offered for download in a thread called UBBCentral Style was the one used here and not something different?

Yea probably.
But it really is the same style just that Rick probably edited it a little over time with the newer releases.
Like I said this style was created for a prior version. So there are a few css properties not populated but should take on default values.
Maybe Rick can export a current style for 7.5.3.
But if he does he probably will place it at ubbskins.com

To get rid of the alternating colors you can edit the style
Control Panel » Styles » Edit or Add a Style
select the "Forum Summary Properties"
There are 26 properties under this heading
If you look just to the right of each property there is a copy link.
Select copy for all 26 properties.
Then you can select the update preview area button as you go to see the change.(if you want to)
When you are done select the update button to save the changes.
Takes a whole 20 seconds.

What is the correct UBB Central Style file to use for v. 7.5.4.1 and up ?

Well the first post by Rick has two attachments. The one for 7.1 is the latest available version.
Which I use for v7.5.4.2.
But you will need to tweak it a bit for some missing css properties like the mouse hover tool. Depends on what features you enable.

Any chance an updated version of this stlye will be uploaded?

The style has been here for quite some time and I don't think any one is willing to update it till after version 8 comes out.

But it does work fine with version 7.5.4.2.
Like I said you just need to use the copy link in some of the newer features in the style editor after importing.

you can take the one here, import it and be very close to what you'd need for 7.5.4.2

bleh, ruben is faster on the draw today