Prevent the app from crashing when no theme is specified

[astorije]

defaults/config.js says:

    // @default  "themes/example.css"

However, without this fix, not setting a theme makes the app crash with:

lodash.templateSources[0]:10
((__t = ( theme ? theme : "themes/example.css" )) == null ? '' : __t) +
          ^

ReferenceError: theme is not defined
    at eval (lodash.templateSources[0]:10:11)
    at /Users/jeremie/Perso/lounge/src/server.js:103:11
    at FSReqWrap.readFileAfterClose [as oncomplete] (fs.js:380:3)

We have been setting default inline like this in other places. Even though it's not optimal, it's better than crashing :-)

[astorije]

It looks like this was not needed after #387, as explained in your comment: #475 (comment)
I was testing this on an outdated version of the code, forgetting the branch I was in was not up-to-date with master...

I'll test and if it's working, cleanup the uses of inline defaults like this one and others.