OS X Yosemite introduced a much flatter, sleeker design. I personally find it great because it takes the focus off of the windows and buttons, making more room and bringing more focus to the content of your documents and web pages. But some people like clearly delineated outlines of buttons and other screen elements, and find the new interface to be confusing. In response to this need, Apple has an option to add high-contrast lines that make the screen easier to read.
To turn the feature on, go to the Apple menu > System Preferences > Accessibility (4th row). With “Display” selected from the left column, check the box to “Increase contrast”. This is a per-user setting that won’t effect other users on the same Mac.