For an example see the screenshot here: https://community.atlassian.com/t5/Portfolio-for-Jira-questions/Jira-Portfolio-Create-Plan-UI-Text/qaq-p/955589
This happens because Portfolio uses a CSS class called "progress". Since "progress" is a common word, it is possible for a custom theme to apply styling for it.
To solve this, Portfolio should use a more specific CSS class. For example "jpo-progress".