The blog post by Gleb Bahmutov provides a tutorial on testing an application that changes a CSS variable to update the background color when a user selects a new color via an `<input type="color">` element. The application uses CSS variables defined in an external stylesheet to manage the color changes, with a JavaScript file handling the event of a color change by updating the CSS variable. The post details a series of tests using Cypress, starting with a simple confirmation that the background color changes upon user interaction, and advancing to more complex tests that spy on and stub DOM method calls to verify the application's internal workings. The tutorial emphasizes the use of Cypress commands such as `.invoke` and `.trigger` to simulate user actions and demonstrates how to verify the functionality by observing changes in the application's state. Additionally, the post provides tips on using tools like the chai-colors NPM module and Sinon.js for more flexible assertions, and it encourages exploring further examples of testing browser and DOM APIs with Cypress.