You seem to be running into https://support.liferay.com/browse/LPS-37433 - that one is supposed to be fixed in the version you're running, but look at the related issues: There's one called "Final Fix":
Some JDK update has changed the way PNG files are processed and Liferay has an offending png image (you might add your own ones). The easiest way to fix this is to understand the issue and change the relevant PNGs manually to some that can be processed by java.
Unfortunately hunting down the png is a bit harder than necessary. As you're developing your theme, you might want to run Liferay in a debugger and configure a breakpoint on any trigger of the exception you see. Then you'll find the relevant filename if you move up the stack and inspect parameter names in the debugger. Or, try the calendar-dropshadow image that's mentioned in the linked ticket first.