Unlike Orkut, why doesn't Facebook allow users to change their homepage theme color, style, etc.?
Do they want people to use their UX or colour strategies?
There are far more important things when it comes to UX for Facebook
A site with 2.5 billion active users needs to constantly rethink things, test them, make them better, add new functions etc. the UX gain is far too small and the impact on the overall site far too big.
How do you measure how users react to different things if one user has a background in magenta and the other user a background in black etc. elements don't work the same on every design.
Also, the global brand identity gets weakened if you let your users decide that Facebook looks different for every user.
I'll point to Reddit as an example of why not to allow users to redesign their pages: it becomes a fragmentation issue. If you allow users to control the look and feel of the software, then you lose control of the experience. Reddit has mostly kept the same, dated design for a number of years, because a cohesive redesign means they will strip subreddits of their designs.
How welcoming is this (notably self-aware) /r/crappydesign subreddit to new users of Reddit?
I'll add that Stack Exchange allows for the customization of colors, logos, and voting buttons per community. Although not available to users, this gives a personal touch but retains control over the vast majority of the UI.
why doesn't Facebook allow users to change their homepage theme color, style, etc.?
Another reason they don't do that is - When users share their problems to FB help centre, their screenshots should look the same. If people used personalized styles, it is possible that it may interfere in correctly identifying and reproducing a particular problem a user is having.
In short: Because it causes hindrance when reproducing a bug