You'll have to add a new CSS class to qa-theme/SnowFlat/qa-styles.css:
And change the font size of the note like this (this class already exists):
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask here, in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.