How do I change fonts & icons in a theme? [duplicate]

I've been finding fonts on the internet and installing them on my system.

What's the easiest way to change my fonts? I would prefer an easy to use graphical way, but answers with command line recommendations are also welcome.

Ubuntu 18.04 and later versions

The latest Ubuntu uses GNOME Shell. You can change the fonts using the "GNOME Tweaks" (gnome-tweak-tool) program.

Install the program using Ubuntu Software:

And change the font and font size.

13.04 - 17.04

For Unity interface users, customisation of fonts can be done using unity-tweak-tool

For 12.04:

For Unity interface users, they have their own tweak-tool. Customisation of Fonts in 12.04 can be done through myunity

In Oneiric (11.10), I wasn't interested in installing GNOME Shell just to change font sizes. This will do it via command-line:

gsettings set org.gnome.desktop.interface document-font-name 'Sans 10'
gsettings set org.gnome.desktop.interface font-name 'Ubuntu 10'
gsettings set org.gnome.desktop.interface monospace-font-name 'Ubuntu Mono 11'
gsettings set org.gnome.nautilus.desktop font 'Ubuntu 10'
gconftool --set /apps/metacity/general/titlebar_font 'Ubuntu Bold 10' --type STRING

The fontfaces are the same as in default Oneiric, but sized down to what looked like much more manageable sizes.

For 11.04 and earlier:

Right click on the desktop and select "Change Desktop Background" and then click the Fonts tab. Couldn't be easier!

(NB: After playing with these setting you may wish to reset your font configuration.)