Add color for downloaded nodes

Author: camelid

src/building/bootstrapping.md

@@ -22,14 +22,15 @@ Compiling `rustc` is done in stages. Here's a diagram, adapted from Joshua Nelso
 [talk on bootstrapping][rustconf22-talk] at RustConf 2022, with detailed explanations below.
 
 The `A`, `B`, `C`, and `D` show the ordering of the stages of bootstrapping.
-<span style="background-color: yellow">Yellow</span> nodes are built with the stage0 compiler, and
+<span style="background-color: lightblue">Blue</span> nodes are downloaded,
+<span style="background-color: yellow">yellow</span> nodes are built with the stage0 compiler, and
 <span style="background-color: lightgreen">green</span> nodes are built with the stage1 compiler.
 
 [rustconf22-talk]: https://rustconf.com/schedule#bootstrapping-the-once-and-future-compiler
 
 ```mermaid
 graph TD
-    s0c["stage0 compiler (1.63)"] -->|A| s0l("stage0 std (1.64)"):::with-s0c;
+    s0c["stage0 compiler (1.63)"]:::downloaded -->|A| s0l("stage0 std (1.64)"):::with-s0c;
     s0c & s0l --- stepb[ ]:::empty;
     stepb -->|B| s0ca["stage0 compiler artifacts (1.64)"]:::with-s0c;
     s0ca -->|copy| s1c["stage1 compiler (1.64)"]:::with-s0c;
@@ -39,8 +40,9 @@ graph TD
     s1ca -->|copy| s2c["stage2 compiler"]:::with-s1c;
 
     classDef empty width:0px,height:0px;
+    classDef downloaded fill: lightblue;
     classDef with-s0c fill: yellow;
-    classDef with-s1c fill: lightgreen, color: black;
+    classDef with-s1c fill: lightgreen;
 ```
 
 ### Stage 0