How to remove light gray borders around content

.entry table, .entry tr td {border:0;}

With the above CSS:
(Note tables are a much older technique for page layout, consider using CSS for all layout).

To discover what CSS is output by your theme, use a web inspection tool such as Firebug: https://getfirebug.com/ , Chrome Inspect Element: https://developers.google.com/chrome-developer-tools/ or Internet Explorer 9+ F12 Dev. Tools: https://msdn.microsoft.com/en-us/library/dd565627(v=vs.85).aspx#csstool

There are others.

When editing CSS, use a Child Theme
https://codex.www.ads-software.com/Child_Themes
Custom CSS Plugin, or Theme provided custom CSS option.
Edits to parent themes are lost on theme update.

Learn CSS: https://www.w3schools.com/css/