FontStyle fs = button1.Font.Style;

        if (Bold.Checked == true)
            fs |= FontStyle.Bold;
        else
            fs &= ~FontStyle.Bold;
        if (Underline.Checked == true)
            fs |= FontStyle.Underline;
        else
            fs &= ~FontStyle.Underline;
        if (Italic.Checked == true)
            fs |= FontStyle.Italic;
        else
            fs &= ~FontStyle.Italic;
        if (Strikeout.Checked == true)
            fs |= FontStyle.Strikeout;
        else
            fs &= ~FontStyle.Strikeout;

        button1.Font = new Font("Tahoma", (float)27.75, fs);

var fs = (Bold.Checked) ? FontStyle.Bold : FontStyle.Normal;
fs |= (Underline.Checked) ? FontStyle.Underline : FontStyle.Normal;
fs |= (Italic.Checked) ? FontStyle.Italic : FontStyle.Normal;
fs |= (Strikeout.Checked) ? FontStyle.Strikeout : FontStyle.Normal;

button1.Font = new Font("Tahoma", (float)27.75, fs);