Since you have no custom CSS yet, I cannot test these in your custom CSS file so I've added the " !important " attribute to all of them to make sure they override the original settings.
This will give you square corners on everything.
#image-preview, #tooltip, #primary-nav ul ul ul, #content .hentry, #hero .edit-link, #footer, #page-header, .default-notice, .jp-audio, .jp-type-playlist, .image-carousel-items .entry-thumbnail, .page-header, .widget, .prev, .next, .jcarousel-prev, .jcarousel-next, .jp-progress, .pagenavi, .purchase-link, .jp-audio, .post-edit-link, .widget .jp-playlist, .widget .hide-content .jp-current-artwork img, .widget .hide-content.playlist-open .jp-playlist {
border-radius: 0 !important;
}
To change the width of the columns, you will have to total the amounts you add to the sidebars (grid 3 and grid 6) and then add that to the width in the first bit of code below. Grid 3 is the two sidebars and grid 6 is the center column content.
#branding, #main, #primary-nav, #footer-container {
width: 960px !important;
}
.grid-3, #site-info, #primary-sidebar, #secondary-sidebar {
width: 220px !important;
}
.grid-6 {
width: 460px !important;
}
What you cannot change the width of is the post image carousel at the top of the center and right columns. That is probably going to be an issue for you visually.