Constant SCHEME_SCRIPT 

pub const SCHEME_SCRIPT: &str = "// The visitor\'s color scheme for these pages: System, Light or Dark,\n// remembered for the whole site. Loaded in <head> so a saved choice\n// applies before the first paint. The control ships hidden and this\n// reveals it, so without JavaScript a page follows the system setting\n// (or the scheme it was generated with) and shows no dead control.\n(function () {\n  var KEY = \"asbuilt-docs-scheme\";\n  var MODES = [\"system\", \"light\", \"dark\"];\n  var root = document.documentElement;\n  var fallback = root.getAttribute(\"data-theme-default\") || \"system\";\n  var mode = fallback;\n  try {\n    var saved = window.localStorage.getItem(KEY);\n    if (MODES.indexOf(saved) >= 0) mode = saved;\n  } catch (e) {}\n  function apply(next) {\n    if (next === \"light\" || next === \"dark\") root.setAttribute(\"data-theme\", next);\n    else root.removeAttribute(\"data-theme\");\n  }\n  apply(mode);\n  document.addEventListener(\"DOMContentLoaded\", function () {\n    var select = document.getElementById(\"scheme\");\n    if (!select) return;\n    select.value = mode;\n    select.parentElement.hidden = false;\n    select.addEventListener(\"change\", function () {\n      mode = MODES.indexOf(select.value) >= 0 ? select.value : fallback;\n      apply(mode);\n      try {\n        window.localStorage.setItem(KEY, mode);\n      } catch (e) {}\n    });\n  });\n})();\n";

The script behind the scheme control, linked in every page’s head.