The phrase "optimistic tone" means that someone is speaking or writing in a positive and hopeful manner, showing belief that things will turn out well. Full definition
One of the report's co-authors has said he believes that it displays «a more optimistic tone about our ability to adapt» to the impacts of climate change than previous reports did. (salon.com)
Central banks strikes a mostly optimistic tone in touting rebound in economic activity in recent months and improvement in the labor market. (fortune.com)
I write most of my previews with a relatively optimistic tone, and I could tell you good things about most of these teams. (sbnation.com)