Here's a simple CSS guide;
In most modern browsers there's now a thing which lets you distinguish elements of a web page. In Firefox based browsers this feature is under web developer -> inspector
On chrome based browsers;
I'm not sure about other browsers but they may have a similar feature if you look for it. Having this and using it helps in making better looking styles.
Another tool which is also helpful is this color rgb to hex tool;
With it you can get the color values to hex easily from a color you like, like if you take a screen and put it into ms paint and use sampling tool on it. Apparently Paint.net has this function in it natively so in the color dialog you just extend it out (more >>) and you can see where the R: G: B: values are and there's also a Hex: value.
Lastly there's these 2 pages;
What I usually start with is to save the page (File -> save page as) on a page which has a style I like. Just make sure it saves the whole page and the files along with it. That way I can open up the folder which contains the webpage files, like the .css files and study/edit them.
On an imageboard if it has a custom style, it usually either has the name custom.css or [nameoftheboard].css
Usually when one starts styling, the simplest thing to start with is to change the background color and text and its best to do the body first. Using the inspector tool in the web browser, you can actually change the css elements while on the page to see how it might look. Simply hover over a part of the page, click on it and the css lines should appear in a window for temporary editing to see how it may look.
Styles will cascade, which means that any styles you add after the styles in the .css document will usually overwrite the prior ones. Child elements inherit the parent elements.
This article explains it better;
If you need help with anything else just ask