Well, from what I know, identity is a big theme. The protagonist's change to having twintails is not just a physical change but also affects their sense of identity. Also, the theme of acceptance might be there. Accepting this new form and the consequences that come with it in the story world. And there's probably a theme of humor as well, given the rather unique concept of the light novel.
The main themes in 'ore twintails ni narimasu light novel' are likely to include self - acceptance. The character has to come to terms with their new state of having twintails. There could also be a theme of adaptation, as they need to adapt to the new situation this transformation brings. Additionally, friendship or relationships might be a theme, as they interact with others in their new form, and how those relationships are affected by the change is an important aspect of the story.
Chapter 308, "Want me to surrender to him? Don't even think about it!", is the clearest early answer because it turns Want's situation into a concrete plot event.
Chapter 465, "I Want Black Truffles", is the clearest early answer because it turns Black Truffle's situation into a concrete plot event. Early on, Zheng Xun urgently mobilized the program's plant expert.. By the end, Maybe we can find some Lingzhi, and then we could earn even more....
Chapter 287, "Saved", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with For five consecutive days, the search and rescue team had not... and closes on Failing to obtain any news after numerous attempts, Lu Xingchao had....
Chapter 346, "This person doesn't seem to like her very much", is the clearest early answer because it turns Seeing Jiang Li's situation into a concrete plot event.
Chapter 172, "Shang Shaojing is Here Too?!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event.
Chapter 616, "I'm Going to Save Sister Xiao Wan!", is the clearest early answer because it turns Sister Xiao Wan's situation into a concrete plot event. The chapter turns that into something concrete by moving from Since the storm had hit that morning, he had been preoccupied and out...
Chapter 755, "You didn't fail to receive the invitation, did you?", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event.
Chapter 534, "This Grand Era is as You Wish", is the clearest early answer because it turns Crown Prince's situation into a concrete plot event. It opens with Pei Ye swiftly tossed all those people far away. and closes on Wasn't this supposed to be Team Q's encampment?!.
Chapter 339, "Today's Windfall Luck!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The models had already stopped their runway walk when they heard the...
Chapter 576, "The Electronic Pickles are Gone!", is the clearest early answer because it turns Ai Xiaochun's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Tonight, Team Q exceptionally didn't cook.