External style sheets (.css files) are superceded by...
Internal styles in the HEAD section of the topic, which are
superceded by...
Specific styles at the location of the content (SPAN tags,
etc.).
PLUS, your browser settings can mess with all of the above.
Somewhere in the first two style areas or the browser lies
the answer. Note also that styles can be contextual: that is, any
SPAN tag that appears as a child of the BODYTEXT class will be blue
underlined.
What you are experiencing is highly unusual, but fixable.
Good luck,
Leon