How to add more colors to SharePoint calendar

Question

What I have:
I am using SharePoint Online from Office 365.
I've created a color coded SharePoint calendar using overlay.

What I want:
I want to use different colors for color coding than those proposed by SharePoint
(The current colors are very dark and not distinctive)

What I already know:
Changing anything in this version of SharePoint is very difficult as it is not supported by Microsoft.
I can't add a script web part in this version (as far as I know but it was checked thoroughly) and I am yet to find out how to add CSS customization without using third party software (which I can't do as I am the admin of only my site, not the organization)

Answer 1

The colors are controlled by the css classes from ms-acal-color1 to ms-acal-color9 stored in Themable/corev15.css. And the color options are based on the site theme.

As you cannot add script web parts or upload custom css files to the site, the only way is to change the look of the site: