Looks like my theme is missing some table styles! I think this is the first time it's been requested though. For now you could add some styles to add a background to the heading row and every other row to help with readability. Here's some example code:

thead tr,
tr:nth-of-type(2) {
  background: #f4f4f4;
}

td {
  border: 1px solid #f4f4f4;
}

I found this workaround on Stack and it worked for me:

Adding this at the beginning of the post:

<style>
.tablelines table, .tablelines td, .tablelines th {
        border: 1px solid black;
        }
</style>

and this after the tables:

{: .tablelines}

Really nice solution @fjolublar, I'm going to add table styles into the latest update. Thanks for highlighting this issue