Struggling with text readability

I am trying to style this pen and I can't quite get the right CSS for the text to make it more readable and not looking overcrowded.

Core CSS:

p{
    color: #000;
    font-size: 16px;
    line-height: 1.25em;
    padding: 0 0 1em 0;
    text-align: justify;
}

I plan to use the improved CSS both for the normal, desktop version and for mobile. Here's a screen of what I think is a great example of readability for mobile.