If we can apply different themes to different charts on same webpage, that will be good. Because if we apply same highcharts theme to all charts that changes all charts color combination (column, pie, bar etc). If it is possible to add a particular theme to particlular chart so that colors of column, bar, pie will be according to defined in theme.
By doing this we can create a single js theme file for all charts, which will contain all same css for all the charts and different theme for colors of column, bar, pie, heatmap etc.