Dashboard > Forums > Settings > Style Tab.
This admin page contains all necessary options to customize wpForo front-end colors and styles. wpForo Classic theme comes with six different color set: default (blue), red, green, orange, grey, dark. You can select color set or customize each color manually using Color Guide.
Using these set of colors, you can choose color style, save it and refresh forum front-end. You may also need to delete all caches and do Ctrl+F5 on frontend in order to re-load wpForo CSS fiels.
You also can manually customize each color using color pickers and Color Guide.
If you need more customization, you can put custom CSS code in according textarea.