https://support.mozilla.org/en-US/kb/userchromecss-js-usercontent-unsupported

To compound that I really have no idea what " I'd like to alter the style of the border and remove the annoying vertical bar on the left" means. Do you mean the one that delineates the change from the folder pane to the "list view" pane or is this something about the silly boxes mail is displayed in in the "card view"

You might want to have a look at the developer toolbox (Ctrl+Shift+I) in the inspector

I am referring to the folder tree. I would like to change the style on the selected / focused folder. Specifically, I'm looking to change the border and remove the goofy block/bar on the left.

I have attached a screenshot which may help.

It's possible to colour the border for selected and hovered folders, but I haven't figured out how to modify the bar at the left of selected folders.

/* folder hover border bg color */
li[is="folder-tree-row"] :hover > div.container  {
background-color: lightblue !important;
border-color: red !important;
}

/* folder selected border bg color */
li[is="folder-tree-row"] .selected > div.container  {
background-color: DeepSkyBlue !important;
border-color: red !important;
}

To hide the border:

/* hide folder border */
li[is="folder-tree-row"] > div.container  {
border: none !important;
}

It's possible to colour the border for selected and hovered folders, but I haven't figured out how to modify the bar at the left of selected folders.

/* folder hover border bg color */
li[is="folder-tree-row"] :hover > div.container  {
background-color: lightblue !important;
border-color: red !important;
}

/* folder selected border bg color */
li[is="folder-tree-row"] .selected > div.container  {
background-color: DeepSkyBlue !important;
border-color: red !important;
}

To hide the border:

/* hide folder border */
li[is="folder-tree-row"] > div.container  {
border: none !important;
}

Thanks! That did the trick.

I used this to remove that silly block to the left of the selected folder:

/* hide idiotic folder highlight thingy*/
li[is="folder-tree-row"] > div.container::before  {
display: none !important;
}