It’s an error of the theme’s developer. I’ll contact him about this. Thanks for creating this question!
Meanwhile, to fix this, in your theme’s style.css, find and delete (or you could also comment it out) this line: font-weight: inherit;
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For future reference, though, you might wnat to post theme-related questions in that theme’s section: http://wordpress.org/support/theme/leaf
THat way, your question would appear on here AND in that support section of the theme. Twice the publicity!
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Side Note: Normally we prefer not to edit the parent theme, but this is an exception . . . because the dev would most likely fix this on the next update of the theme (if any).
This means if we add additional lines to custom CSS or child-theme CSS to fix this, it would potentially cause problem on the next update (when the dev rolls out his own fix) if you forget to remove it.
