Navigate posts within the same category

Dear imax Team
I’d like to navigate the posts within the same category only. I have adjusted the get_adjacent_post function in the function.php in line 355 and 356 by setting the first Parameter from false to true.
But this has no Impact. The navigation is still through all Posts.
I appreciate a lot if you can you guide me in this matter.
Best Regrads
Michael

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