If you use Firefox, there is a very good guide to the Firefox Developer Tools here.
It's much more comprehensive than anything I could write here.
The most useful tool will probably be the Page Inspector.
The Select an element functionality should allow you to determine which containers on your web page are the ones that you want to modify via CSS.
You can then add the CSS rules to the appropriate containers (via ids or classes) in your theme's 'custom.css' file (for example 'showkase/_themes/maribo/css/custom.css'.
A reference guide for CSS can be found here.
The section on Selectors (ids and classes) will likely be useful.
I hope this helps and points you in the right direction.
(Other browser have their own, similar, developer tools. Please check your own browser's support for details.)