Interesting to know! I switched to Open Sans 300 for now and looked at it from the phone, hope that makes it a little easier on the eyes!

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.

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.