self hacked code if your themes are somehow lost within your filesystem

get-all-posh-themes.ps1

$response = (curl "https://github.com/JanDeDobbeleer/oh-my-posh/blob/eed32ec55a392928cab6cfb52b0311677febd089/themes")

$content = $response.Content

# "/JanDeDobbeleer/oh-my-posh/blob/eed32ec55a392928cab6cfb52b0311677febd089/themes/M365Princess.omp.json"
# JanDeDobbeleer/oh-my-posh/blob/main/themes/1_shell.omp.json
$results = $content | Select-String '([a-zA-Z0-9\._\-]+)\.omp\.json' -AllMatches

$array = $results.Matches.Value

# Remove duplicates
$array = $array | Get-Unique

$api_url = "https://api.github.com/repos/JanDeDobbeleer/oh-my-posh/contents/themes/"

foreach ($theme in $array) {
    # use api to get the content
    $url = $api_url + $theme
    
    curl $(gh api $url --jq .download_url) -o $theme

    mkdir -p $HOME/.oh-my-posh/themes
    mv $theme $HOME/.oh-my-posh/themes
}