a layman’s introduction into locating css selectors without browser inspection tool (although it is much easier with the tools):
you can find those css selectors by just looking at the html output of the corresponding section in the browser – ‘view source’
if you are using the default ‘custom menu’ widget in a child theme of Twenty Twelve, the output should be someting like:
<aside id="nav_menu-2" class="widget widget_nav_menu"><div class="menu-side-menu-container"><ul id="menu-side-menu" class="menu"><li id="menu-item-3146" class="menu-item menu-item-type-custom menu-item-object-custom current-menu-item current_page_item menu-item-home menu-item-3146"><a href="https://localhost/wordpress">Home</a></li>
<li id="menu-item-3026" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-3026"><a href="https://localhost/wordpress/about">About</a></li>
<li id="menu-item-2675" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-2675"><a href="https://localhost/wordpress/gallery-page">Gallery Page</a></li>
<li id="menu-item-2676" class="menu-item menu-item-type-taxonomy menu-item-object-category menu-item-2676"><a href="https://localhost/wordpress/topics/uncategorized">uncategorized</a></li>
</ul></div></aside>
from that you can find the common css selector for each menu link; for example .menu-item
to make that specific for the sidebar, add .widget-area
before it, and style with whatever fixed or fluid width, padding, background-color, and rounded corners:
.widget-area .menu-item { }