<snip> So all your CSS tweaks should go into the very last CSS file that is called. Calendar css would be found in the calendar module. But all module CSS is called before your theme css, and you don't want to hack the calendar module. In your theme, go to its .info file and see the order of the css files called and find the last one (it's typically style.css or local.css). Put your changes in there. Clearing cache is always a good thing. </snip>
I'll look in .info, had not thought of that, thanks.
My observations/tests so far. --Clearing Cache does not clear the files in /default/files/css. --I do not know where to look for discussion on this /default/files/css and the erasing of files therein. Do they regenerate with modified css code if erased? --It seems that the css in /default/files/css is th last called, according to firebug. --css in this file overrides the css elsewhere in some but not in all cases. --Altering css code in these files has no effect on the display. --Altering css in the calendar module has no effect on the display while the /default/files/css is present. --Altering the css in /files/css with firebug changes the display but making the same alteration the file does not. --None of the above seems to apply with other modules and themes.
More testing Thanks to all who help Roger