A PPA is a software repository for a specific programs. It's easy to distribute this way, since you can automatically upgrade the software.

First we need to enable PPA so we can add it later, type it in terminal:

$ sudo apt install software-properties-common

Then add the Elementary Tweaks PPA:

$ sudo add-apt-repository ppa:philip.scott/elementary-tweaks && sudo apt-get
update

Finally, install it:

$ sudo apt install elementary-tweaks

You can start the software from the Applications launcher, or from Software Settings.

This is the first tab, where we can change the look of Elementary OS. However one thing you need to keep in mind, that Elementary itself is heavily modified. It has its own window manager, so you may need to try many themes, before you find one that actually works. If you want to do that, you can go to Gnome look, and download some GTK themes, then extract it to your HOME directory, inside .themes, which is a hidden directory. Elementary OS use a unique window control button order, since there is no minimize option and close and maximize is on the opposite sides.

These are the usual options, default for the UI, document is for written stuff, monospace for terminal and where we use fix width fonts, and last, the titlebar is for the window titles.

We can disable animations if you use a weaker machine, although I think you shouldn't use Elementary OS if you don't have a decent machine.

And we can change the animation speed for close, open, snap, minimize and for workspace switch.

Only one option, for changing the max sound volume. To be honest this should be in the Sound option.

Files itself have so limited options, that the terminal based ranger has more. We can turn on single click, restore tabs (which is very useful) and change the date format.

Just so we can change that the category view is the default for the launcher.

We can change the background color, and some useful ones, like natural copy paste, and that we get an alert when we paste some some command with sudo.

Some options for sticking window on the top, or don't instantly start the playback.

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As you can see Elementary Tweaks, has many little options, and I am hopeful they will continue to work on this application.