How Can I Change The Backround Color Of A Joomla Template? - In The Css File/the Html? How?

I’m not sure what a joomla template is, but the css code to change the background color is as follows

body {background-color:#XXXXXX;}

where XXXXXX is the hex code of the color, or you can use the color name - or, for divs especially, you can use ‘transparent’, though that won’t necessarily work for the body background.
There are other attributes you can use as well - mostly for background images, however.
background-image:url(URL OF IMAGE);
background-position:NNN NNN;
background-repeat:REPEAT or NO-REPEAT;
background-attachment:FIXED or SCROLL;

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