I wrote the content of my project in Google doc.
Copy paste the content from google doc to the robohelp. Now I am not able to change the font family and styling in the robohelp.
I tried to do it from the css file but the changes are not reflected.
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<moved from using the community>
Most likely you have pasted in a bunch of inline styling from the Google doc into the RH topics. You'll have to clean out that styling in RH to enable your css to work. Copy/pasting from another source always benefits from "washing" it through a text editor like Notepad first.
You haven't said which version of RoboHelp you are using. Please always start a thread with this information.
Look at one of the topics in source/html view (depending on version) and you will see a lot of inline styling applied, that is overriding your CSS.
Probably the easiest solution is to copy the content in author/design view into a text editor and then replace the content in your topic with the text from the text editor. That will lose all the inline styles and you can apply the CSS styles in RoboHelp.
My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.