A little help and advice please

hello
i am shell i have an autism forum which styles are CSS,i have no clue where to even start to look for help.
i have autism/learning difficulties so grasping things can take a longtime for me so please if i dont understand can you please expalin simply for me to be able to process.
i am not here for handouts i really want tolearn CSS to change my forum style as the ones they have are not very good at all,i know what i have in mind but do not know how to go about even starting a style sheet.i made a banner out of the background of the file below but its not very good as i dont know html or css,the forum owners have added mods that as changed it so only CSS codes work,I dont know where to even start i am a complete novice i can create styles though in my admin panel but first i need to know where to start,how to code and how to check it,I am so sorry i am a blank book with CSS
thank you so much for your responseI make visuals/pictures for autistic children who cannot speak,they are expensive to buy so i make them for free,i also offer support and advice as having autism myself and having autistic children i can give a bit of an insight.i hope thats still ok i dont have a blog.

http://aisforautism.co.uk/ my style that needs changing
I would like to learn how to make a style for my message board.
this is what i want to make
http://www.awesomestyles.com/phpbb3-styles/view/rainbow
as i have an autistic spectrum site this would be wonderful
thank you in advance