Yes, you can manually install a theme.
Be sure to read the instructions provided by the theme author...
You may want to first use
Bald Brick suggestion as that way they easy to install
and remove.
Bald Bricks way also gives you the option of
modifying the actual colours.
On the
Scheme Tab I've selected the Frothy Pink (gulp

) theme. See first screenshot

If you now choose the
Colors Tab (see next screenshot) it gives greater control over which colour you can apply to "individual" parts of a theme.
May want to experiment with a Theme you
don't want to use and it will give you an idea of how it works