Yoo hwan, There seems some issue with the bar chart background. I will investigate more on it.
Meanwhile as a workaround you can add a Grid layout first and then add the bar chart in it.
Then you can set the background color of the cell (row/column) where bar chart is added.
Yes, it is possible to have different color for each graph. You just need to create different css class for each graph and map it as per your requirement i.e. add the name of the css class in the graph settings.
For e.g. you can create css classes timeseries1, timeseries2 and so on for different timeseries charts.
I hope it helps.