how do i add customized text style in MicroStation

Having created your Text Style in a *.dgnlib file, in order to share these Text Styles within the dgnlib, with other files, as indicated by MaryB you will need to set the Configuration Variable MS_DGNLIBLIST or the other alternative is to use MS_DGNLIBLIST_TEXTSTYLES. 

See also

(+) MicroStation CONNECT Edition Configuration Variables - MicroStation Wiki - MicroStation - Bentley Communities

Building DGN Libraries (bentley.com)

Having created your Text Style in a *.dgnlib file, in order to share these Text Styles within the dgnlib, with other files, as indicated by MaryB you will need to set the Configuration Variable MS_DGNLIBLIST or the other alternative is to use MS_DGNLIBLIST_TEXTSTYLES. 

See also

(+) MicroStation CONNECT Edition Configuration Variables - MicroStation Wiki - MicroStation - Bentley Communities

Building DGN Libraries (bentley.com)