You probably need to edit the .css files for that specific theme.
Look in the \theme\serenity\style folder on your Moodle server (not via the web browser) for the .css files of that theme.
Also, purge your Moodle theme cache before you do anything else. Go to Site administration/ Appearance/ Themes/ Theme selector and click the Clear theme caches button.
Inch by inch, slowly I turned....
OK, I found and opened the CSS file for Serenity but I really did not find anything to gave confidence to be that which I could change for my purpose.
Taking an educated guess, I tried: Line-Height on line 272
#custommenu .yui3-menuitem-content {
color: #fff;
font-weight: 800;
line-height: 20px /* Originally: 30px;*/
}
but other than that, nothing seemed to talk to me.
Cleared the Cache and no changes were noted.
Next?