how do i add customized text style in MicroStation

What is the exact version no. (e.g. v10.x.y.z) of MicroStation that you are using? Use key-in VERSION to see the version no. in the MicroStation Message Center, or Help→About MicroStation in the backstage.

See this blog that shows you how to obtain MicroStation's version number.

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Ramogale Selebogo said:

How do I add customized text style in MicroStation CONNECT?

Open the Text Styles dialog.  Add a new style.  Edit the new style.

Ramogale Selebogo said:

i have tried (dgnlib not working) 

When editing a Text Style, the file you are editing must be open for write.  Open your DGNLib using MicroStation.  Create and edit your Text Style in that DGNLib.  When you open a normal DGN file, the text styles in that DGNLib are available automatically.

Also, the DGNLIB file must be included in the list defined by the MS_DGNLIBLIST configuration variable.
Creating the file is only the first step. After that, you need to edit your configuration so the MicroStation knows to include it.

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)