The easiest way to do this is to bump up all text, or specific sections of the UI (posts for example).
html { // all text in Discourse
font-size: 15px; // default is 14px
}
.container.posts,
.d-editor-container {
font-size: 15px; // increase text in posts and the post composer by 1 font size
}
There’s no general concept to increase only text that’s currently 14px to 15px (you could do it, but it’s a lot of hand-picking individual elements to style). Generally it’s not the best idea to only increase paragraph text because suddenly the paragraph text might be larger than some header text, which kind of messes up the emphasis/hierarchy.