CSS Code for standard font style and size

We do not need custom CSS for this, as these options are present under look and feel

No unfortunately they're not. This is everything I see under the look and feel - style menu.

Since you are using some static theme, you are not able to view that options, so in that case use below CSS in custom CSS
.Skin .QuestionText, .Skin .QuestionBody {
  font-size: 18px!important;
font-family:Arial!important;
}

    I've used this code to change the labels on mobile as they weren't fitting on the screen, hopefully it's useful:
@media (max-width: 480px) {
ul.labels {
    word-break: break-word;
    font-size: 6px;
    }
}