Note: This was originally published in 2015; I've updated it with a minor change required for Mojave, and clarified a bit of the text.
macOS includes—and enables by default—translucency, which gives you 'wonderful' effects such as this in Calculator:
This is just one example; lots of other apps (Mail and Messages, to name two) contain panes that become grossly distorted by background color bleed-through. I'm not sure who at Apple (Marketing?) thinks this feature is good for productivity , but I find it completely distracting.
As a result, I turn off translucency on every Mac I own. You can do so yourself in System Preferences > Universal Access > Display. Just check the Reduce transparency* box, and you won't get any more bleed-through. (You'll also get a solid Dock, and perhaps the world's ugliest Command-Tab task switcher. Such is the cost of usability.)
* It's ridiculous that Apple calls this transparency, which is defined as "the condition of being transparent," and being transparent means being see-through, clear, invisible, etc. This is clearly translucency, or "allowing light, but not detailed images, to pass through." But I digress…
However, when writing for Many Tricks or Macworld, I often need to take screenshots. And because most users won't disable translucency, I prefer to take those screenshots with translucency enabled, so that they're closer to what most users might see. That means a trip through System Preferences to toggle the checkbox, which gets annoying after the second or third time you've done it.
There had to be an easier way—and after some missteps, I eventually found it.