Use custom styles in sidebar

[Michal Kozubík]

Can I use custom styles for the Google Sheets Sidebar? Or will this not pass the addon review for the marketplace publishing?

I know there is stated

Use the add-ons CSS package in your HTML service pages

But I would like to use UI5 framework that comes with its own styles. And I don't want to create completely new theme for UI5 just because of this.

[Alan Wells]

Yes, you can use custom styles.  But, you still need to adhere to the guidelines.  If the custom style does not adhere to a certain guideline, then you would need to make modifications.  I'm guessing that most add-ons are rejected at some point, but you can correct the issue, and still get your add-on published.  It's difficult to know whether you can get your add-on accepted without any modification to the theme that you want to use.  The only way that you'll know for sure is to submit your add-on.  They do want to see consistency in the style when you show new content, like a new dialog is opened, or a new tab is displayed.  If the theme is based on material design, then there probably won't be any issues with general spacing or layout.  Make sure not to display internal error messages, or long error messages, or long explanations in messages.