Currently mass_genre_update for MAL only grabs the basic genres below:

Action - Genre ID: 1

Adventure - Genre ID: 2

Avant Garde - Genre ID: 5

Award Winning - Genre ID: 46

Boys Love - Genre ID: 28

Comedy - Genre ID: 4

Drama - Genre ID: 8

Fantasy - Genre ID: 10

Girls Love - Genre ID: 26

Gourmet - Genre ID: 47

Horror - Genre ID: 14

Mystery - Genre ID: 7

Romance - Genre ID: 22

Sci-Fi - Genre ID: 24

Slice of Life - Genre ID: 36

Sports - Genre ID: 30

Supernatural - Genre ID: 37

Suspense - Genre ID: 41

But it would be really great if it also pulled the additional Themes and Demographics. Visible on the screenshot.

These have commonly been used in Collections config and all have a MAL Genre ID.

ie Adult Cast - (mal_id: 50) - https://myanimelist.net/anime/genre/50/Adult_Cast

Genre IDs at this time go up to 83, with the possibility of more being added.

THEMES

Adult Cast - Genre ID: 50

Anthropomorphic - Genre ID: 51

CGDCT - Genre ID: 52

Childcare - Genre ID: 53

Combat Sports - Genre ID: 54

Crossdressing - Genre ID: 81

Delinquents - Genre ID: 55

Detective - Genre ID: 39

Educational - Genre ID: 56

Gag Humor - Genre ID: 57

Gore - Genre ID: 58

Harem - Genre ID: 35

High Stakes Game - Genre ID: 59

Historical - Genre ID: 13

Idols (Female) - Genre ID: 60

Idols (Male) - Genre ID: 61

Isekai - Genre ID: 62

Iyashikei - Genre ID: 63

Love Polygon - Genre ID: 64

Love Status Quo - Genre ID: 74

Magical Sex Shift - Genre ID: 65

Mahou Shoujo - Genre ID: 66

Martial Arts - Genre ID: 17

Mecha - Genre ID: 18

Medical - Genre ID: 67

Military - Genre ID: 38

Music - Genre ID: 19

Mythology - Genre ID: 6

Organized Crime - Genre ID: 68

Otaku Culture - Genre ID: 69

Parody - Genre ID: 20

Performing Arts - Genre ID: 70

Pets - Genre ID: 71

Psychological - Genre ID: 40

Racing - Genre ID: 3

Reincarnation - Genre ID: 72

Reverse Harem - Genre ID: 73

Samurai - Genre ID: 21

School - Genre ID: 23

Showbiz - Genre ID: 75

Space - Genre ID: 29

Strategy Game - Genre ID: 11

Super Power - Genre ID: 31

Survival - Genre ID: 76

Team Sports - Genre ID: 77

Time Travel - Genre ID: 78

Urban Fantasy - Genre ID: 82

Vampire - Genre ID: 32

Video Game - Genre ID: 79

Villainess - Genre ID: 83

Visual Arts - Genre ID: 80

Workplace - Genre ID: 48

DEMOGRAPHICS

Josei - Genre ID: 43

Kids - Genre ID: 15

Seinen - Genre ID: 42

Shoujo - Genre ID: 25

Shounen - Genre ID: 27