I tried some changes mentioned here, but they didn't work for me on version 2.8. Here are the changes I tried.
- change essential.css file which is located in theme/essential/style directory (Posted by Lee Taesang on Friday, 22 May 2015, 7:33 PM)
- the CSS rule you want is located in <moodle>\theme\base\style\question.css (Posted by Joseph Rézeau on Saturday, 11 February 2012, 4:38 PM)
I was able to implement the first change listed above but not the second. Regarding the second, the theme I'm using, called More, did not have a file called question.css in the directory that Joseph mentioned.
I did find a solution, though. I copied the css mentioned by Lee and pasted it into the Custom CSS box for my theme.