"padding" is for for the inside of a container.
"margin" is what you are looking for.
To specifically target the bottom margin, use "margin-bottom"
Every table uses a single standard table wrapper of .t_outer and .t_inner. These can be customized within the style editor. The left/right columns use the same table wrappers as every other table wrapper in the software. This allows designs to remain constant across all pages.
If you wanted to add an additional margin of 100px to the bottom of your tables, you would insert "margin-bottom:100px;" to .t_outer or .t_inner. within the "General CSS Properties" group of your custom style, within the Style Editor.
NOTE: You've previously posted 7.5.x code to another thread, so If you are are coming from a 7.5.x style, and you're having table display issues, you may have duplicated items for t_outer and t_inner, causing a "double border/margin/padding." Those border/margin/padding items within the t_outer group can be placed within the t_inner group. Before submitting your changes, click on the "Update Preview Area" button to confirm everything looks as you expect it to.