Mac's tend to have App Icons that are all over the place. Here's a quick little trick to modify them and make them consistent.
First we need to disable SIP to change some system icons. This is pretty simple, but read about it here
With this done, we can now modify apps like Finder, Messages, Mail, or any Apple Apps.
You can manually modify app icons, but it's a pain. LiteIcon gives you a nice UI to drag/drop icons and search for apps.
This is the most fun part, picking an Icon Pack! You can spend hours here, and find the one that best fit your taste. I normally look at DeviantArt for some. The Icons I use are Monomite.
Now just search for the apps in LiteIcon, drag and drop the icon on the app, hit "apply" and log-out to see the updates applied.