If you only want to change the font and concise the two lines text into one line then you only have to make changes into your mailer file. Just decrease the font size and remove the next line tag. After making this change do not forget to refresh the cache.
Caching is the process in which various variable are stored in the cache of the system. It makes the data extraction very fast. Cache is stored in magento_root/var/cache folder. To refresh cache go to System cache management panel and refresh all the cache. You can also manually clear the cache by deleting all the content of magento_root/var/cache folder.