Css question – I’m confused.

Hello Folks, I’m very much a newbie to WordPress. Just yesterday I installed a blog on a website I built for my newborn son. Kudos to the people who have built this product, install was a breeze and I was up and running within an hour or two. I was able to modify the CSS to match the look of the existing web site but the calendar days where there has been posts don’t show up until you mouse over them. I read the support documentation concerning modifying the CSS but I just can’t seem to find the value to change so days with posts are visable. Any help would be appreciated. You can view the blog at https://www.declanmatthewryan.com