You mean
<link href="../Css/a.Style.css" rel="stylesheet" />
in a.about-us.html and
<link href="Css/a.Style.css" rel="stylesheet" />
in index.html
You can do absolute in both html files:
<link href="/Pages/Css/a.Style.css" rel="stylesheet" />
or if Pages is the root folder:
<link href="/Css/a.Style.css" rel="stylesheet" />
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…