It can be found in constants inside /src/constants.ts
But here is the list:
const DEFAULT_STYLES = [
"abap",
"algol",
"algol_nu",
"arduino",
"autumn",
"base16-snazzy",
"borland",
"bw",
"colorful",
"doom-one",
"doom-one2",
"dracula",
"emacs",
"friendly",
"fruity",
"github",
"hr_high_contrast",
"hrdark",
"igor",
"lovelace",
"manni",
"monokai",
"monokailight",
"murphy",
"native",
"nord",
"onesenterprise",
"paraiso-dark",
"paraiso-light",
"pastie",
"perldoc",
"pygments",
"rainbow_dash",
"rrt",
"solarized-dark",
"solarized-dark256",
"solarized-light",
"swapoff",
"tango",
"trac",
"vim",
"vs",
"vulcan",
"witchhazel",
"xcode",
"xcode-dark",
];