Calendar Widget Appearance

Hi,
(I’m new to WP, so if this is a silly question, sorry.)
I added the calendar widget to my site and the lines (that separate days) look funny. They are missing between Tuesday and Wednesday. The line between Wednesday and Thursday extends down past any other line. On my cell phone, all the lines are correct except that there is an extra box for Wednesday, under the last line for the month. Is there a way to clean this up? Thanks!