Sorry if it's the wrong section.
How much knowledge do you need to start coding a mybb theme , HTML/CSS?
I am very good at HTML, But i just do not know how much knowledge i need for CSS
You can learn CSS by studying tutorials, or get some free templates and "just change something and see what happens", then repeat over and over. CSS is really easy with some experience*.
*Make sure you remember to check IE to see if it is messing up something that looks great in other browsers.
Making a good theme is also about making something that is a bit more than a colour change.
Try looking about on the Internet at sites you admire, asking yourself what it is you like about it and then using those concepts in the theme. It is not a question of stealing a whole design wholesale (as I am sure you know) but taking what you like and adapting it to fit the theme.
Example:
This site
http://proxart.co/category/art definitely influenced this forum display:
http://blackcanvas.net/Forum-News-Suggestions-Feedback
They don't look anything like each other, but I know that the one inspired the other.
So what's a good site to learn CSS?