This problem is likely to be this launchpad bug which is specific to using the theme Radiance in Unity-2D.
If it is then there is a workaround you can try.
Some before and after screen-shots will help here with this method:
how did I do that
Themes are described in text files in the folder
/usr/share/themes. Specifically themes use what is termed Content Style Sheets to define what they should look like.
Since these are text files - you can edit and change these easily.
First you have a choice - you can edit the files directly - or via a copy in your own home folder.
If you edit the files directly, then an update to the theme will overwrite all of your good work ... but then again - it may be the update you are looking for.
Lets assume you want a user specific theme. The following commands copies the Radiance theme into your home folder.
Lets open a terminal and copy and paste the following:
mkdir -p ~/.themes
cp -r /usr/share/themes/Radiance ~/.themes
What you want to edit is in this section of the file:
You need to add
text-shadow: none; to this section (i.e. between the
} curly brackets) so that it now looks like:
Logout and login to see the change take effect.