Sorry, maybe I didn't explain well.
I meant the file directory where the block is located: your_moodle_site/blocks/styles.css
But this block is part of the moodle core and do not include any style. So it means that use the defaults theme styles. If you want a different style than the rest then you must create your own adding the .block_myoverview block class (you have a very good sample of styles for blocks in blocks.css included in Adaptable) . Or if you want to add all the borders then just apply border with the desired styles to the class .block_myoverview