@numericcitizen Here is a simpler way to accomplish something similar. It's not exactly what you asked for, but it's easier and non-intrusive. Add the following to your custom CSS in micro.blog. Don't need to edit a custom theme.

.post-preview { border-bottom: 1px dashed lightgray; margin-bottom: 30px; }

You'll need to play with the exact values to get whatever you want. You can use color variables as well (like in the rest of the theme).