Your devices software is at fault ,
and/or you overrode the standard (linked) files
Each xHTML can have a different CSS Linked. Or multiple (Calibre standard is page.css, stylesheet.css)
If you link in another AND the styles overlap, confusion reigns (the last defined style prevails)
Multiple
VALID Stylesheets work fine. RMSDK is noted for being prone to barking on the slightest error.
It works (8 stylesheets via EPUB merge) FINE on RMSDK
I Modify_EPUB most books. FINE on RMSDK
I Polish on occasion. FINE on RMSDK
I even use the Embed Metadata. FINE on RMSDK
Run Reports from the Editor. you may have a double define
Validat you stylesheets with W3C