In Sigil, Code View:
First in one of the html files that has the CSS code, copy the code and paste into notebook.
Paste the following at the top of the notebook file:
Code:
@namespace h "http://www.w3.org/1999/xhtml";
Save the file as stylesheet.css.
Right click the Styles panel - Styles folder and select Add Existing items. Browse to and open stylesheet.css you've just created.
Back into Code View:
Select and copy the style information of the cover page file. Paste into notebook temporarily.
Use Find/Replace as follows:
Look in 'All HTML files'
Check 'Regular expression' and 'All'.
Code:
Find: (<style)[^</style>].*(</style>)
Repl: <link href="../Styles/stylesheet.css" rel="stylesheet" type="text/css" />
Finally replace the original CSS into the cover page html.