In this world there are humans have developed the technology to allow for interstellar travel, accomplished by light speed engines. However, this has created a problem. Is there any way for those who live on the vastly different planets can stay genetically similar to humans on Earth? Conditioning like people living on a freezing cold planet having a higher tolerance level against the cold is fine; but evolving to have less efficient muscles for more body heat wouldn't be allowed. Is there any way this idea can be implemented in this world?
The only natural way to keep populations from genetic drifting is regular interbreeding. Fortunately, the mechanism for this would take hundreds of thousands of years before you'd diverge enough to cause breeding issues. For example, the peoples that made it across the Pacific by whatever route were separated from their Asian ancestors for at least 13,000 years, and maybe as much as 50,000.
There are a few known unnatural ways of doing it, but they all involve identifying an "ideal center" for genetics and testing a new child at some point in development. You can pick the point at which this occurs for optimal dystopic effects on your story.
Half of all pregnancies are currently non-viable and result in an immediate reset of the reproductive system. These are called "chemical pregnancies," and generally result in a slightly heavier than normal period. If you induce a virus that prevents divergent pregnancies from taking hold, you will keep your population within "human norm", but you run the risk of a population eventually becoming barren because their genetics has drifted up against one of the virus-imposed walls.
There are more than enough stories about cultures that test babies at birth and throw away the unwanted ones. Such high-tech barbarism might make for excellent drama. If the testing took place later, you could guarantee that there was a lot of cheating going on.
Alternately, you could require one in a thousand women to get knocked up by a traveling God King (or his near descendants) who was the epitome of ideal genetics. If these children were given favorable economic and social treatment, that would maintain a standard.
Addendum: In response to @JBH's comment, I want to note that H. sapiens sapiens (that's us) were cross-breedable with H. sapiens neanderthalensis. That's right, neanderthals interbred with our ancestors as recently as 45k years ago, even though they diverged from us more than a half million years ago. The upshot of this is that your interstellar civilization has at least one orbit around the Milky Way before you have to worry about it. Put another way, the stars you're visiting will diverge more than the populations' genetics will.
Note that this presumes that the speed of genetic variation matches that of our cradle. Given higher population counts and a bad habit of encountering radiation, you might run into that problem a bit sooner, but not in the time-scales described in Asimov's Foundation trilogy.