Looking for the correct way to override #content styles

Hello,

I am using the Twenty Ten theme and I would like to change the default styles for tables.

Here’s what’s in the styles.css :

styles.css

#content table {
    border: 1px solid #e7e7e7;
    margin: 0 -1px 24px 0;
    text-align: left;
    width: 100%;

I try to reproduce this solution :

https://wordpress.org/support/topic/how-to-overrideremove-parent-css-setting-in-child-css?replies=4

So here’s what I add in my Simple Custom CSS plugin :

#content
{
  .table: auto;
}

or .table width: auto;

But the #content style still shows up (I use the firefox element inspector)

I can create my new styles just fine with
.something { }

Is my syntax wrong or am I missing something?
Obviously, I don’t want to directly edit the styles.css and I would like to avoid creating a child theme, as my edits will be minor.

Many thanks.