No indentations for the options in the select box in an inlined table (Django MPTT)

Question

I created many-to-many relationship with the tables "Category", "Product" and "CategoryProduct" which is the middle table between "Category" and "Product":

"models.py":

from django.db import models
from mptt.models import MPTTModel, TreeForeignKey

class Category(MPTTModel):
    name = models.CharField(max_length=50, unique=True)
    parent = TreeForeignKey(
        "self", 
        on_delete=models.CASCADE, 
        null=True, 
        blank=True, 
        related_name="children"
    )

    class MPTTMeta:
        order_insertion_by = ["name"]

    def __str__(self):
        return self.name


class Product(models.Model):
    name = models.CharField(max_length=100)
    
    def __str__(self):
        return self.name


class CategoryProduct(models.Model):
    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    product = models.ForeignKey(Product, on_delete=models.CASCADE)

    class Meta:
        unique_together = [['category', 'product']]

Then, I registered "Category" and "Product" with the inline "CategoryProduct":

"admin.py":

from django.contrib import admin
from mptt.admin import MPTTModelAdmin
from .models import Category, Product, CategoryProduct

admin.site.register(Category, MPTTModelAdmin)

class CategoryProductInline(admin.TabularInline):
    model = CategoryProduct

@admin.register(Product)
class ProductAdmin(admin.ModelAdmin):
    inlines = [CategoryProductInline]

Then, I added some categories and as you can see, the options in the select box of "Parent" have indentations:

But when I tried to add a product, as you can see, the options in the select box of "Category" don't have indentations:

Is it possible to give indentations to the options in the select box of "Category"? If possible, how can I do this?