If you build your Typo3 website using templavoila, you can include a CSS stylesheet in the header of your HTML template. Templavoila provides the header mappings in the template definition to choose which header tags to keep included in the template. However, since templavoila release 1.3.7, the include order of header tags has been changed. Nowadays. these header tags will be placed before any TypoScript-based includes in the generated html output. As a result, if you want to overwrite the css definitions of an extension (e.g., powermail) in your custom stylesheet, this is not possible by using the templavoila header mappings due to the ordering.
To solve this problem, instead of using the templavoila header mappings for your css include, you can do this with TypoScript. The code below includes the css file after the includes specified by any Typo3 extensions
# Include CSS File
page.includeCSS{
cssfile1 = fileadmin/templates/_styles/myfile.css
}