ActiveHL WebExtension

manifest.json

{
  "manifest_version": 2,
  "name": "ActiveHL",
  "version": "1.0",
  "description": "Put the active tab in red.",
  "permissions": ["theme","tabs"],
  "background": {
    "scripts": ["styling.js"]
  }
}

styling.js

var querying = browser.tabs.query({currentWindow: true, active: true});
querying.then(updateStyle, onError);

function onError(e){
  console.error(e.message);
}

function updateStyle(){    
    var theme = {
        "colors": {
          "accentcolor": "#black",
          "textcolor": "white",
          "tab_selected": "red"
      } }
    browser.theme.update(theme);
}

Licence: CC-0