Graham bemethavinci Css is a Cascading Stylesheet System
Styles in css files linked below the main css will add to or override those set in the main css
you can just create another css file to hold custom classes or extra styles for existing classes
and add a link to that file below the bootstrap links
ie <link rel="stylesheet" href="/my_stylesheet.css">
or even just create a <style>...</style> in the _headtags below the main css links
look at my test page I added a basic background image for the 'body' and another for the 'panel' class in the temporary style tag in the twig
the .description-image class controls the size of the thumbnail in the post list (only using my twig as its not a standard class)
and the @media rule makes it larger on larger screens
http://baretest.xtgem.com/newblog

if you want to create a decent custom web page without just copying from other sites
you need to study everything CSS / javascript / twig / jquery / html and whatever else you want to use