We have a list of open bugs. Lots of them low hanging fruits, fixing CSS and such.
Please help fixing them!
- create a branch
- do the fix,
- add a changelog entry
- commit with a message referencing the bug number and push
- if not already: become Plone Core Contributor, see https://plone.org/foundation/contributors-agreement
- go to github and send a pull request (PR) referencing the original bug number
Be part of the exciting Plone community 
For more information on how to contribute read http://docs.plone.org/develop/plone-coredev/index.html