Hello, if you go under the series properties and change the colors it will over ride what is coming out of the theme. It is not super straightforward as they show a grey color when you look at them but once you change them it overrides the theme color. Keep in mind this is for an XY but I would think it will work for time series as well.
Down side of this is that you cannot leverage style classes as the colors expect a HEX code. I setup a UX tag provider with all my chart colors set in memory tags then simply bind in the stroke, fill colors on charts.
Check out this link.
https://forum.inductiveautomation.com/t/perspective-xy-chart-line-color-feedback/22989
Frank