How to Detect If Current Terminal Supports 16-Color or 256-Color in Go?

As mentioned in "Determine whether the current Linux console supports 256 colours?" (and that is on Linux, the same portability issue would exist on Windows)

As an example of non-portable option, you could use curses (on Linux), as discussed here and implemented in curses.go:

Looking at tput colors and tigetnum('colors') (which return numbers like 1, 8, 256, or greater) are more along the lines of feature testing, which is less hacky and more future-proof.

func Init(theme *ColorTheme, black bool, mouse bool) {
    {
        in, err := os.OpenFile("/dev/tty", syscall.O_RDONLY, 0)
        if err != nil {
            panic("Failed to open /dev/tty")
        }
        _in = in
        // Break STDIN
        // syscall.Dup2(int(in.Fd()), int(os.Stdin.Fd()))
    }

    C.setlocale(C.LC_ALL, C.CString(""))
    _screen = C.c_newterm()
    if _screen == nil {
        fmt.Println("Invalid $TERM: " + os.Getenv("TERM"))
        os.Exit(2)
    }
    C.set_term(_screen)
    if mouse {
        C.mousemask(C.ALL_MOUSE_EVENTS, nil)
    }
    C.noecho()
    C.raw() // stty dsusp undef

    if theme != nil {
        C.start_color()
        var baseTheme *ColorTheme
        if C.tigetnum(C.CString("colors")) >= 256 {
            baseTheme = Dark256
        } else {
            baseTheme = Default16
        }
        initPairs(baseTheme, theme, black)
        _color = attrColored
    } else {
        _color = attrMono
    }
}