What needs to be fixed

There are several bits about the resulting popup from HTMLHelper::calendar that feel very out of date and in need of some fixes.

Why this should be fixed

It unfortunately looks like it's been unchanged since Joomla! 2.5 despite having the brand new Bootstrap buttons in the footer

How would you fix it

1. Make it so that the table .calendar-container table uses a margin: auto to center itself. I initially tried using a width: 100%, but that appears to conflict with someone else and ruins the entire layout.
   
2. Remove the class .nav from the td tag inside thead. There is a conflict of style where they are being forced to display: flex, affecting the alignment of their internal objects. If the class .nav is absolutely needed for something like a11y, a style override should be added to calendar.css that resets back to display: table-cell.
   
3. Stylize the time dropdown menu. It is completely unstyled currently, and sticks out compared to everything else. I added the classes form-select and form-select-sm to stylize and edited the html of that line to remove the empty td and redistribute that wasted space.
   

Side Effects expected

I don't forsee any negative side effects, but I'm sure someone more familiar than me would be able to correct me.

Why not do a PR myself? I literally can't figure out how to, and I don't seem to be smart enough to figure it out by myself.