Okay, first you need Notepad++
Then, get a manifest.json file (I'm not sure how to make one, but someone said: A json file is nothing but javascript object notation. You can create this file just by using any of your editor like notepad and save it with the extension .json. Thats it.
Try saving it with double quotes. Like this - "your_file_ name.json"

Then you need images you're going to use, of course. You have to crop them and resize them yourself for it to fit in your screen (which means there are separate themes for diff. screen resolutions).

You have to create a folder which has in it the manifest.json file and another folder named "images" for the images you're going to make.

This is the site on how to make them:
[link]

This is an example of it:
[link]

And by the way, DO NOT USE COPY-PASTE because it will get an error. Also you have to mind the alignment of the lines, like how far the indent is.

Then when you're done, go to Google Chrome, go to Tools, then Extensions.
Check the Developer Mode box on top.
Then use "Pack Extension". In extension root directory, click Browse, look for the folder that contains both the manifest.json file and the images folder, then click Pack Extension. If it says there's an error that states Line or Column whatever, it means there was a mistake in the manifest.json file.

Also, make sure you put 2 in the Version instead of 1, if you cannot see the image in the new tab after using the theme.

I just want to ask, have you guys studied coding already? Cuz if you haven't this might get a little difficult, but don't worry! With the right amount of patience, you'll get the hang of it!

Ask me if there's something you don't understand! ^_^