I really like the look and functionality of this plug-in, however, trying to get the colors to match my theme proved difficult -- I have seen a lot of questions regarding the css properties associated with the month/day flag on the upcoming events widget. I never found a solid answer, so here is what I came up with.
Here are the css properties I changed to match the flag color to my theme style. Just remember that without creating a child theme, these will revert back to default with any plugin update.
Find general.css in "wp-content --> themes-ai1ec --> vortex --> css --> general.css"
Edit the following :
.ai1ec-agenda-widget-view h3.ai1ec-date-title - border property (line 1703)
.ai1ec-agenda-widget-view .ai1ec-month - background property (line 1732)
You may also wish to edit the event text color using:
.ai1ec-agenda-widget-view .ai1ec-event a - color property (line 1765)
Hope that helps!