Looks like you only changed the font that will be loaded from Google Fonts; styles.css is still using Lato:

  * {
    font-family: 'Lato', sans-serif;
  }

For clarity, the complaint is primarily about weight 300, not about the font family. Open Sans is a little better than Lato at weight 300, but it’ll still look quite poor on many devices.

Check out practical typography for some quick and easy tips to improve the typography: https://practicaltypography.com

Personally, I would ditch the header typeface too. You can surely find something which is fairly unique while still being legible.