Chrome Developer Tools: How to find out what is overriding a CSS rule?

Use the Computed Style panel of the element inspector. Expand the property of interest to see the list of applicable rules, and which one won.

You can simply look at the ones with the same name which aren't striked out, remember the listing is by importance.

Or you can view the computed styles. They will be the actually applied styles.

crtrl + shift + c and inspect the element. Then find the style without a line through it, in the box in the down right corner.

the override is in most cases at the top (and without a line through it, as this style is the "winning" one).