Gabriel Morin's post is good, but tile-qt doesn't build anymore with those instructions and, though the blog post he linked to (mine, by the way) still works, it's no longer the best solution I know.
For people who want something better than the "clam" theme, PySolFC includes a Clearlooks theme for Ttk and I wrote another blog post which explains how to install it.
Here are the relevant parts of it, adapted for a Lubuntu user (which I now am):
# Get the theme
sudo apt-get install pysolfc
# Copy it into your user profile to not rely on PySolFC's presence
mkdir -p ~/.local/share/tkthemes
cp -r /usr/share/games/pysolfc/themes/clearlooks ~/.local/share/tkthemes/
# (optional) Remove PySolFC now
sudo apt-get remove pysolfc
sudo apt-get autoremove
# Add it to Ttk's search path
echo "export TCLLIBPATH=~/.local/share/tkthemes" >> ~/.xsessionrc
# Set the theme
echo "*TkTheme: clearlooks" >> ~/.Xresources
# Test it in this specific terminal window
xrdb -merge ~/.Xresources
git gui # gitk's "this isn't a repo" dialog is always ugly
# ...and then log out and back in to apply .xsessionrc for the whole desktop
There are some widgets which are still ugly (the menu bar and the
SHA1 ID: text, which needs to be patched in gitk to be themable) but I don't know enough Tcl/Tk to patch things.
(And I've heard rumors that the menu bar wasn't made themable because it's delegated to the OS to draw on Windows and MacOS)