add theme settings

https://echarts.apache.org/en/theme-builder.html
https://echarts.apache.org/en/download-theme.html

- [ ] support for standard themes from echarts and themes created in echarts builder
- [ ] to create your own themes in ggdsl-echarts

Start with the ECharts theme builder and download-theme links in the issue to understand the required theme formats. No repository file, test, or entry point is named, so inspect the project structure to locate theme handling before defining the scope. Done should cover standard and builder-created ECharts themes, plus creating custom themes in ggdsl-echarts.

Written by the indexing model from the issue text.