function fish_load_theme --description "Load a Fish prompt theme"
    if test (count $argv) -ne 1
        echo "Usage: fish_load_theme <theme>" >&2
        return 1
    end

    set -l theme $argv[1]
    set -l theme_dir $__fish_config_dir/themes/$theme

    if not test -d $theme_dir
        echo "fish_load_theme: Theme '$theme' not found." >&2
        return 1
    end

    if not test -f $theme_dir/prompt.fish
        echo "fish_load_theme: Theme '$theme' has no prompt.fish." >&2
        return 1
    end

    source $theme_dir/prompt.fish

    if test -f $theme_dir/right_prompt.fish
        source $theme_dir/right_prompt.fish
    end
end