ComixCore is both a site and a WordPress Theme!
The theme is basic, but easily customizable. It has WP Customizer support allowing you to quickly change the colors of several areas of the webpage without using html. Everything is open source and available on GitHub if you want to edit anything. Actual functionality for displaying comics and other features for creators will be added by plugins. (See below.)
The ComixCore-Comics plugin adds comic displaying capability. The plugin works with any theme and allows for page-by-page or vertical scroll viewing. There is also series support if you have more than one comic. Additional features are planned as time permits and the plugin is open source if you want to add your own.
I have ideas for a few other plugins as well when I am able.
The theme and plugin are both open source. Downloaded them from GitHub.