Alright Thanks mag3737
The mean problem lies with the HTML output. I did go through the Help section on "Appendix A: Configuring XHTML and CHM output". I have copied the commontlr.css file and created a custom one on my desktop which I named it "commontlr_custom.css".
I removed everything inside, leaving only the <p> element because I wanted to do some testing first, to see if the custom file works. So I tried setting everything embeded within <p> to have RED font color
.p {color: red}

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Next, I created a Concept Topic, type in some text, which is within the <p> tag.

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After that, I displayed it in Multiple HTML file. But before that, I clicked on [Configure Output] first, and under Deliverable Types, I set the stylesheet path to my commonltr_custom.css.

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Finally I run the file. I even Right Click and View Source to see if the stylesheet is linked.

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Well as you can see, the text isn't red. I have also tried doing the same thing to <bold> element, but nothing changes. Can anybody point out the mistake I have done?
Regards