Example of CSS customization in Blogger.

Having slept too much, I wanted to have separators for my html tables.
Google explains here how to add custom css.

I introduced an arbitrary class name so it isn't applied to the blog theme itself, only inside posts when explicitly stated.

table.withborder {
  border-collapse: collapse;
}

table.withborder td {
  border: 1px solid #624A93;
}

Then in the HTML editor simply annotate:

<table class="withborder">

Implicit approach

To avoid adding withborder everywhere, leverage the class used by blogger (there is no guarantee it won't break at some point, though).

div.post table td 
{
  border: 1px dashed RebeccaPurple;
}