Do you ever have players complain about restrictions/changes based on theme?
I follow my vision with the game I want to present, and those who aren't interested will pass. I'm not a jerk about it, but it's my baby, and they'd better respect her.
For example, the stand alone adventure I'm currently running is about questing knights. I have a couple of goals for this adventure (in addition to having an enjoyable experience, and maintaining an appropriate theme). One of those goals is that I want to see how certain different ways to represent knights in shining armor in 5e compare to each other. So when I sent the player primer to those who had expressed interested I explained that there were four choices for you character's class, and each player would choose a different one. Paladin (Devotion), Cleric (War), Fighter (Battle Master or Champion), and Bard (Valor).
They were told that at least half of them should be human, but that half-elf, half-orc, and aasimar are available also. (I got 2 humans, a half-elf, and an aasimar.)
When we got together for our session 0, I let them decide who would play what.
Player 1 mentioned that things seemed a bit restrictive on character options for knights (in other words, he accepted the parameters, but thought I might have overlooked something). He mentioned Storm Cleric's for instance. I explained to him that the reason that wouldn't work was because the domains represent certain orders, and that order (I explained who they were) didn't work well as a knight in shining armor. He was fine with it. Another player asked about Vengeance Paladins, and I shot it right out of the air--explaining to him that it didn't really fit the concept, and also explaining who the Vengeance "Paladins" are in my settings. He laughed about it and moved on. The first player brought up a concept for a ranger in heavy armor and asked about Magic Initiate (Druid). Since no one else had claimed the bard (the least vital character in my comparison), I went ahead and approved the ranger. It's a pretty cool character with a lot of flavor and a fun backstory. Would I have let it sub for the fighter, paladin, or cleric? No. The player also had ideas for any of the other classes, so if his ranger didn't fly, he would have picked something else.
So complaints? Not really. Discussions? Yes. Thematic integrity? Always.