if you want to switch ereaders, try this ereader. it allows you to switch the screen from white with black letters to black screen with white letters.
it is called allreader. it also supports a lot of book format versions, quote from link below.
AllReader+ supports TXT, HTML, RTF, FB2, DOC, DOCX, ODT, SXW, ABW, ZABW, CHM, PDB/PRC (PalmDOC, zTXT mode 1) and TCR formats.
AllReader+: Powerful Reading Tool :: Windows Mobile Pocket PC :: Products