From The Document Foundation Wiki
Go to▸ ▸ ▸ and make sure the choices are
- Set Icon style in the User Interface section near the top to Colibre (SVG). Use of SVG (scalable vector graphics) provides perfect rendering when icons are resized.
- Set Icon size to Small for the Toolbar, Notebookbar, and Sidebar.
If you are using Ubuntu or another Linux distribution and the icon style drop-down list does not show Colibre as one of the choices, go into the distribution’s package manager and install the libreoffice-style-colibre package.