Added
- Added a set of plot themes that are full-RGB versions of the [builtin
plotext
themes](https://github.com/piccolomo/plotext/blob/master/readme/aspect.md#themes).
Each is prefixed with `textual-` followed by the plotext theme name.
- Added `textual-design-dark` as a Textual-friendly theme designed for use
with dark mode.
- Added `textual-design-light` as a Textual-friendly theme designed for use
with light mode.
Changed
- Change the default dark and light mode themes in `PlotextPlot` to use
`textual-design-dark` and `textual-design-light`.