If you are a space administrator, then you can change the theme from the Global default. The Global Look and Feel looks very pretty but is not ideal for in-space navigation. It removes the tree-style list of all pages in the left sidebar, so it's a bit hard to figure out how to change. Follow the steps below to change the sidebar to the classic and simpler Documentation Theme.
At the bottom-left corner of the space page, click Space Tools > Look and Feel.
Select the the radio button next to Documentation Theme, then click Confirm.
Click the Colour Scheme tab.
Hit Select on the Global Colour Scheme, and your color scheme will switch back to what it was previously.
If you need to get back to Space Administration, click Space Tools on the bottom right corner.