Although its not a native Avast solution, I'm getting a fairly good dark rendering of the forum by using a web browser add-on . Only "light" section is the banner heading that has the search window and the "main menu" bar, the rest of the topic and reply pages are dark. Details: Firefox browser v87.0, although earlier versions also worked; Dark Reader add-on v4.9.29, default settings; Windows 10 v20H2, Dark Colors setting.