Of course..
But overriding CSS is not like overriding PHP...You can just override the specific rule(s) instead of the whole file.
CSS file loading sorting on the page is very important. The CSS file that loads into page in the last position (considering CSS are loaded within <head>...</head>, this is the CSS file that is closest to </head>), the last CSS file can override any rule(s) from the above CSS files, because it is being loaded in the last. It is most powerful one.
So, pull the CSS rules that you want to override from the theme1.css, theme2,css or theme3,css and put them to the last CSS file (or any CSS file that is being loaded after theme1.css) and re-write the rule as you wish..
If theme1.css say:
.someThing {
font-size:11px;
}
then put a CSS rule to the last CSS file to override it as follow:
.someThing {
font-size:12px;
}
so the font-size will be overriden. I hope you understand what i am saying.. CSS overriding is a basic web development. Try google.com or youtube.com to find more tutorials about it.
Regards.
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