added defaultColorTheme and defaultSizeTheme to ReprProvider

src/mol-repr/representation.ts

src/mol-repr/structure/representation/ball-and-stick.ts

src/mol-repr/structure/representation/carbohydrate.ts

src/mol-repr/structure/representation/cartoon.ts

src/mol-repr/structure/representation/molecular-surface.ts

src/mol-repr/volume/direct-volume.ts

src/mol-repr/volume/isosurface-mesh.ts