Buttons (and other elements) appear “glossy”

Hello,

Buttons appear different when using this theme.

By default, when not using this theme, they appear as they are here (solid color):

https://www.w3schools.com/bootstrap/tryit.asp?filename=trybs_button_styles&stacked=h

But when using this theme, they have a gradient/glossy look (not sure how to include a screenshot on here).

How can I remove this look and just use the default look of bootstrap buttons?

Thanks