-- Instead "css" write your own css code
-- Save script to /etc/astra/mod/css.lua
-- Restart Astra
-- Press Ctrl + Shift + R in your browser to reset cache

astra_storage["/mod.css"] = [[
css
]]

astra_storage["/mod.js"] = astra_storage["/mod.js"] .. [[
(function() {
    $.head.addChild($.element("link")
        .addAttr("rel", "stylesheet")
        .addAttr("type", "text/css")
        .addAttr("href", "/mod.css")
        .addAttr("media", "all"));
})();
]]