If I understand, your directory structure is as follows:
siteroot main math css
You want to set the link to the stylesheet in / main / css from / main / math / index.html.
There are two solutions: the easiest way is to use the absolute path:
<link rel="stylesheet" href="/main/css/basic.css" type="text/css" media="screen" />
Absolute paths are the best solution and will cause less headache in the future. I also do not know any flaws.
Alternatively, you can use ".." to move around the directory
<link rel="stylesheet" href="../css/basic.css" type="text/css" media="screen" />
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