If I’m understanding properly, it should already inherit default user-agent styles, but if you want to explicitly, just specify a font-family ( styles jacked from Stackoverflow stylesheet )
textarea {
font-family:Consolas,Monaco,Lucida Console,Liberation Mono,DejaVu Sans Mono,Bitstream Vera Sans Mono,Courier New, monospace;
}
Specify the more specific (monospace-specific) font families first, and if the user agent doesn’t have that available it will keep trying them until the end, in which case it would default to the default user agent font family for that element, if any was specified.