GTK has some good looks by default. However, it can look better. In the list below are five of my favorite themes.

Themes are packages with CSS code to style the dash, launcher, and panel. If you use the GTK+ theme on Chrome, themes can even style your browser.

Note: This article has been updated with eight new themes! Click the link to read the new article. The new themes are suggested by the Google+ Ubuntu and GNOME communities.

Arc

Pictured: My current desktop with Arc Dark GTK and Shell theme. I have modified Arc with a transparent header. The icons are numix circle.

Arc is one of the more popular GTK themes, and rightly so. It comes with a window theme and a shell theme, and it has three color variants. This is my favorite theme, and I use it almost exclusively.

Light

To use the light theme, select Arc in the Theme > GTK+ section of Tweak Tool. (Or however you do it in your platform.)

Dark Headers

To use the dark headers theme, select Arc Darker in the Theme > GTK+ section of Tweak Tool. (Or however you do it in your platform.)

Dark

To use the dark theme, select Arc Darker in the Theme > GTK+ section of Tweak Tool. (Or however you do it in your platform.)

Shell theme

If you have the user themes extension, you can select Arc or Arc Dark as your Shell theme in Tweak Tool > Themes.

How to Install Arc

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Main distributions that work with Arc are

Arch Linux and Arch Linux based distros
Ubuntu 15.04, 15.10 and 16.04
elementary OS Freya
Debian 8
Gentoo
Fedora 21 - 24
openSUSE 13.2, Leap 42.1 and Tumbleweed

Derivatives of these distributions should also work. To download the solid theme without transparency, change every command url from /arc-theme.list to /arc-theme-solid.list.

###+ Ubuntu and flavours

sudo sh -c "echo 'deb http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04/ /' >> /etc/apt/sources.list.d/arc-theme.list"
sudo apt-get update
sudo apt-get install arc-theme

+ openSUSE

Support for Tumbleweed, Leap 42.1, and openSUSE 13.2. Run all commands as root to install.

Tumbleweed

zypper addrepo http://download.opensuse.org/repositories/home:Horst3180/openSUSE_Tumbleweed/home:Horst3180.repo
zypper refresh
zypper install arc-theme

Leap 42.1

zypper addrepo http://download.opensuse.org/repositories/home:Horst3180/openSUSE_Leap_42.1/home:Horst3180.repo
zypper refresh
zypper install arc-theme

openSUSE 13.2

zypper addrepo http://download.opensuse.org/repositories/home:Horst3180/openSUSE_13.2/home:Horst3180.repo
zypper refresh
zypper install arc-theme

+ Fedora

Works on 23, 22, and 21. Run the commands as root or it won't work.

Fedora 23

cd /etc/yum.repos.d/
wget http://download.opensuse.org/repositories/home:Horst3180/Fedora_23/home:Horst3180.repo
yum install arc-theme

Fedora 22

cd /etc/yum.repos.d/
wget http://download.opensuse.org/repositories/home:Horst3180/Fedora_22/home:Horst3180.repo
yum install arc-theme

Fedora 21

cd /etc/yum.repos.d/
wget http://download.opensuse.org/repositories/home:Horst3180/Fedora_21/home:Horst3180.repo
yum install arc-theme

+ Debian 8.0

echo 'deb http://download.opensuse.org/repositories/home:/Horst3180/Debian_8.0/ /' >> /etc/apt/sources.list.d/arc-theme.list 
apt-get update
apt-get install arc-theme

+ Arch Linux

Arch users can install from AUR.
aur.archlinux.org/packages/gtk-theme-arc/

Xenlism Minimalism

Pictured: My desktop with Xenlism Minimalism GTK and Shell theme.

Minimalism has a transparent panel, and it looks very good. However, my main issue with it is that it is very bright and it styles some applications to a unreadable font. This is easily fixed with some css magic, however.

Shell theme

If you have the user themes extension, you can select Xenlism Minimalism as your Shell theme in Tweak Tool > Themes.

How to Install Xenlism Minimalism

Main distributions that work with Xenlism Minimalism are

Arch Linux and Arch Linux based distros
Ubuntu 15.04, 15.10 and 16.04
Debian 8
Any *Nix machine

Details for each platform are below.

+ Ubuntu and flavours

sudo apt-key adv --keyserver keys.gnupg.net --recv-keys 90127F5B
echo "deb http://downloads.sourceforge.net/project/xenlism-wildfire/repo deb/" | sudo tee -a /etc/apt/sources.list
sudo apt-get update
sudo apt-get install xenlism-minimalism-theme

+ Arch Linux

Install with pacman.

sudo nano /etc/pacman.conf

add config section to /etc/pacman.conf

[xenlism-arch]
SigLevel = Never
Server = http://downloads.sourceforge.net/project/xenlism-wildfire/repo/arch

Update System and Install Xenlism Minimalism.

sudo pacman -Syyu
sudo pacman -S xenlism-minimalism-theme

+ Anything else

Works on most *nix systems with wget, curl, or git.

Install with wget

bash <(wget -qO- https://raw.githubusercontent.com/xenlism/minimalism/master/INSTALL/online.install)

Install with curl

bash <(curl -s https://raw.githubusercontent.com/xenlism/minimalism/master/INSTALL/online.install)

Install with git

cd /tmp;git clone https://github.com/xenlism/minimalism.git;bash /tmp/minimalism/INSTALL/offine.install;

Super Flat

Pictured: My desktop with the Super Flat Shell theme. Shown with Super Flat Remix icon pack.

Super Flat is an update of the theme Super Flat Remix, fixing lots of issues with text coloring and such. Super Flat adds lots of great effects to your desktop, such as the shadows on my left dash panel (first screenshot). It comes with only a shell theme, and no GTK theme, so try this shell theme out with Arc, it looks very good.

Shell theme

If you have the user themes extension, you can select Super Flat as your Shell theme in Tweak Tool > Themes.

How to Install Super Flat

Super Flat will work with any machine with GTK and a theme folder. Just download the zip and unzip it to your theme folder. (This is usually /usr/share/themes/). To make this theme work, you have to use the directory from the inside the download, it will not work with the main folder.

Uranus

Pictured: My desktop with Uranus GTK and Shell theme.

Uranus is a nice darker theme, and it has rather nice icons. What ruins it for me is the bad font choices and the cramped text in applications. (See my first screenshot.) However it still is a nice theme, and I rather like the ui.

Shell theme

If you have the user themes extension, you can select Uranus as your Shell theme in Tweak Tool > Themes.

How to Install Uranus

Uranus will work with any machine with GTK and a theme folder. Just download the tar and unzip it to your theme folder. (This is usually /usr/share/themes/)

Yosemite Shell

Pictured: My desktop with Yosemite Shell GTK and Shell theme.

Yosemite Shell is a nice theme for people used to Apple design: blurred top panel, transparent elements, and a different dock. I don't use Yosemite Shell mainly because I want my Linux to look like Linux, not like mac, but its up to you how your computer should look and Yosemite Shell is a good choice.

Shell theme

If you have the user themes extension, you can select Yosemite Shell as your Shell theme in Tweak Tool > Themes.

How to Install Yosemite Shell

Yosemite Shell will work with any machine with GTK and a theme folder. Just download the zip and unzip it to your theme folder. (This is usually /usr/share/themes/). You have two options with Yosemite Shell; transparent panel or blurred panel. They both look pretty similar, transparent panel's panel adjusts its color to your background.

transparent panel

blurred panel

This is part of my Ubuntu collection.
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Do you like these desktop themes? Do you hate them? Drop a line in the comments below with your favorite desktop theme, and I'll be sure to take a look.