Misc tweaks (#19)

* typo?

* use transparent background for dark-friengly imgs

Author: benbovy

docs/earth/raster.md

@@ -17,6 +17,7 @@ kernelspec:
 alt: Alt text
 width: 200px
 align: center
+class: dark-light
 ---
 ```
 ```{grid-item}

docs/earth/xdggs.md

@@ -17,6 +17,7 @@ kernelspec:
 alt: xdggs logo
 width: 200px
 align: center
+class: dark-light
 ---
 ```
 ```{grid-item}

docs/earth/xvec.md

@@ -32,7 +32,7 @@ Learn more at the [xvec](https://xvec.readthedocs.io) documentation.
 A more general definition is "a data cube that contains geometries as variables (e.g. moving features or time-evolving shapes)".
 ```
 
-Xvec's use of custom indexes is exciting because it illustrates how a new Index can help define a new data model --- _vector data cube_: "an n-D array that has either at least one dimension indexed by a 2-D array of vector geometries".
+Xvec's use of custom indexes is exciting because it illustrates how a new Index can help define a new data model --- _vector data cube_: "an n-D array that has either at least one dimension indexed by a 1-D array of vector geometries".
 
 1. Indexing using geometries and associated predicates is supported using `.sel`
 1. A new `.xvec` accessor exposes additional querying functionality.