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With Node.js installed, run npm install and npm run build:dev to compile and run your changes locally. To create a package for publishing, use npm run build.
The CSS is built using Stylus. If you'd like to contribute, you should first install Node.
You can then edit lib/extras/_theme.styl or whatever theme you are changing, then run npm run css.
Some of the selectors in the CSS are very contrived. I found it was the only way to select what I wanted in a document made of tables within tables with barely any classes. I tried to use class selectors wherever I could, but it's mostly unavoidable to go down a long chain of selectors. If you spot any ways to improve any of the selectors, please make a pull request and I'll be glad to take it in!
If you want to add a new module, please edit lib/defaults.json and add a descriptive key for your module (it will be used as the actual module name in the settings panel) and increment the settings_version key. If the module depends on another module to run, please also make sures to add it to requirements properly.
Please keep your new module disabled by default if it's a change that could overwhelm some users. There is also some code in place that will notify the users of the presence of a new module whenever their installation of the extension is auto-updated.
For the actual code, create a JavaScript file in lib/modules with the same name as the module, and subscribe to the settings object through the module's key (take a look at lib/modules/open_links_in_new_tabs.js for an example). This will make the code in your module execute only if the setting is enabled from the interface. NOTE: don't forget to add your JS file to manifest.json.