There are plenty of reasons to use a custom icon pack. Maybe you want your icons to match your totally awesome theme. Maybe you want all your icons to look consistent. Maybe you just want the apps on your new phone to look like they did on your old phone. Maybe you want to disguise your apps so your co-workers won't get into anything you don't want them to.
Whatever your needs, there are icon packs out there for you. Here's how to use them.
Before you can apply an icon pack, you need a launcher that supports icon packs. Most pre-loaded launchers, the app that hosts your home screen and app drawer, do not support icon packs. Some like Samsung TouchWiz and HTC Sense Home allow you select an icon pack from their proprietary theme stores. TouchWiz and Sense Home do not allow you to use a Google Play icon pack, so if you're looking to use one, you'll need to grab a new launcher.
In your icon pack-supporting launcher, the option to apply your icon pack to all of your apps usually lies in the launcher's settings, though they seem to be in a different place for each one. In Nova Launcher, icons are under Look and Feel. In Action Launcher, icons are under Display settings. In Apex Launcher, icons are under Theme settings.
Once you find where icons are hiding in your particular launcher, you're usually given a list of installed icon packs to choose from, plus the original icons, usually referred to as System or Default. Selecting one of these will apply new icons to all of the apps on your home screen and your app drawer, unless you have applied an individual custom icon to it.
What if there's an app that isn't included in your desired icon pack? What if you want to use a different icon in the pack than what is automatically set - such as the color variant icons in Glim? What if you want to use your own image as an app's icon? That's where setting individual icons come in.
In most launchers that allow you to set individual app icons, you long-press the app in question on your home screen and select "Edit." Action Launcher will bring up the icon edit menu after you long-press an app. You'll be given a list of icon packs to choose from again, as well as a few suggestions for app icons.
To set a custom icon from a pack, tap the pack you desire. You'll be given a long list to find the icon you're looking for within the pack, and tap to select it. To set an icon from an image, select one of your gallery apps and find the desired image. Crop your image as needed before setting. I recommend PNG images as they support transparency. As you can tell at this point, there's going to be a bit of trial and error to get just the right look — but that's why you're theming in the first place!
Now that you know how, jazz up your app drawer! Quit letting your icons clash with your theme. Give your phone a little class with some Golden Icons, or let out your inner geek with Monsterbal; just get a bit Random and mix things up!