Widgets

Hello everyone,
I’m a starting webdeveloper, enjoying it a lot. So yesterday i got this brilliant idea to start and learn to code websites in WordPress. I started with a very simple design and it is going very well, it looks good and all functions seem to work except the plugins.

Well, they work, but i have no idea how to custumize the look of plugins and widgets. I don’t see any css-file? Can anyone help me with this?