Phil Tableau highly recommends you use the same versions of Desktop and Server to avoid problems just like this. Best solution: upgrade Desktop to 9.1.
here's my two cents as well!
Published workbooks can only be rendered in fonts that are installed on the server. Ie - if you've used something like Helvetica Neue LT in Tableau Desktop and that font isn't also available on the server, the server will use the 'next best' fit (most likely Arial).
'If you're using a "custom font" (ie basically anything other than the stock standard Times New Roman, Arial etc) - you will need to ensure that this font is also installed on the server in question. Unfortunately, if you're working with Tableau Online - you don't have the option to do this, so you'll have to change the font that you're using in Tableau Desktop.
Unfortunately *fonts* is one of the few things that doesn't get packaged up when we send workbooks to server.
Thank you for your input. What ended up solving this problem for me was to delete the calculated field I was using in the dashboard to apply conditional color formatting and then add it back. Then, delete the published version of the dashboard and re-publish it. Everything looked fine after that. Eventually we plan to upgrade the desktop too.