Well good news, you can have it without using outdated and insecure software! Here's some info on how to customize your browsers and some resources to do so. PLEASE NOTE: these tutorials are designed with Windows OS in mind.
Customizing Firefox is surprisingly simple. This method also seems to work with forks of Firefox (I personally use Waterfox).
For my browser I use the Modern Modoki theme created by soup-bowl. The read-me on the github page has pretty clear instructions, but this tutorial is project agonstic and should work with most custom CSS Firefox themes.
To do this, you need to open a new tab and type "about:config" into your address bar. Once you get to that page, paste this into the search bar: "toolkit.legacyUserProfileCustomizations.stylesheets"
Make sure it is set to "true" if it's not already. You can change the value by clicking the sideways arrows on the far right.
This is also pretty simple and similar to the first step. Open a new tab and type "about:support" into the address bar. This should take you to a troubleshooting information page. Under the "Application Basics" table, there should be a row labled "Profile Folder". There will be a button that says open folder and a file path. Click the button and there's your profile folder.
This exact process can vary a bit project to project, but usually all it requires is for you to download the project .zip, copy the "chrome" folder from the .zip file into your profile folder from the previous step, and restart your browser. That's it.
If nothing changed, make sure you have any baked in Firefox themes disabled. Hit ctrl+shft+a to open the extensions and themes tab. The only enabled theme should be the system theme.
If the theme changed but doesn't look quite right, repeat the steps and make sure to double check the read-me for whatever project you're getting the files from.
Still not working? It might be your version of Firefox. Frequently whenver Firefox makes any kind of visual update, it will break CSS themes. The project you're using may list a preferred version. If not, let the creator know there's a bug!
Unfortunately, Chrome is not so easily modified (and frankly, I wouldn't recommend using it at all). For base, out-of-the-box google chrome the most I can offer is a Chrome 50 theme from the chrome webstore.
However, if you want to get adventurous (and away from google's prying eyes) I would recommend Supermium. It offers the aero-glass style as well as some other community created themes. It's built off of chromium, so it's the same bones as the browser you already know, and it runs on Windows XP! Pretty neat, right?
Something to note about chromium based browsers though is that most of them simply aren't as secure as google Chrome. Generally on the internet today, as long as you aren't clicking shady links or downloading weird files, you should be fine. But, if you're here, you're probably delving well under the surface of the clearnet. So, I'd recommend utilizing this guide for hardening chromium browsers.