Hi,
If the custom theme is applied to the page where web content is display its will be available to web content you can access by using ThemeDisplay object
Css <link rel="stylesheet" href="${themeDisplay.getPathThemeRoot()}/css/custom.css">
JS - <script src="${themeDisplay.getPathThemeRoot()}/js/myscripts.js"></script>
You can refer your current theme files from template using ${themeDisplay.getPathThemeRoot()}
if you want to access other theme from webcontent then you can access
Css <link rel="stylesheet" href="/o/your-theme-name/css/custom.css">
JS - <script src="/o/your-theme-name/js/myscripts.js"></script>
You can refer your other theme files from /o/your-theme-name/
