The reason it’s not doing anything is that the second border statement here is overwriting the new one, you should only have the first:
border: medium none;
border-left: 6px solid #497ca7;
But let’s step back a moment: I strongly recommend that you don’t edit the style.css file in your theme directly, otherwise your changes will be overwritten when the theme is updated.
An easy way to add custom CSS is to install the Jetpack plugin and activate the Custom CSS module. You’ll then add your custom CSS in the new stylesheet editor that’ll appear in your dashboard, under Appearance > Edit CSS.
As alternatives, you could either install a standalone custom CSS plugin, or create a child theme.
Once you add the code senaom provided above to your child theme or custom CSS area, it will override the code in your theme and should take effect.