Perfect. Unzipped Calendar_all.
js.zgz and modified the values of OPACITY to 100 for all of them and voila calendar completely legible again. I realize this "feature" will get re-enabled at the next upgrade, but at least for now my users are happy. And maybe that bug will be released in 5.0.5 and it won't be an issue.
To recap and the solution:
1. cd /opt/zimbra/jetty/webapps/zimbra/
js
2. gzip -d -S .zgz Calendar_all.
js.zgz
3. Search for the following setting: ZmCalColView._OPACITY_APPT_NORMAL
4. Change the values of all the _OPACITY's to your preference (I used 100 for them all to comletely remove the dimming "feature")
5. Save this File
6. gzip -S .zgz Calendar_all.
js
7. Clear browser cache, logout of Zimbra, close browser, reopen browser and try looking at your calendar again. Problem should be resolved.