Where did you place the custom styles?
Those should always be in your theme or a separate plugin if you’re going the filter hook route.
I worked according to your wiki – I put it in the themes folder, and then it disappeared
My guess, is that you used one of the themes that gets updates via the WordPress update system. and didn’t use a child theme.
If you use a core them you expect to update relatively frequently then a child them is always a good idea as that way you can have your custom tweaks (like the M Chart Themes) in a spot that will never be overwritten by the WP update system.
Could I create a plugin of the custom style?
Absolutely.
You’d have to set it up a bit differently of course. You’d use the m_chart_chart_args filter hook:
https://github.com/methnen/m-chart/wiki/Action-and-filter-hooks#m_chart_chart_args
Closing due to lack of followup from OP.
