Add link from themes.md to referenced code block

Just for convenience

Author: timabell

THEMES.md

@@ -18,4 +18,4 @@ Notes:
 * rgb colors might not be supported in every terminal. 
 * using a color like `yellow` might appear in whatever your terminal/theme defines for `yellow`
 * valid colors can be found in tui-rs' [Color](https://docs.rs/tui/0.12.0/tui/style/enum.Color.html) struct. 
-* all customizable theme elements can be found in `style.rs` in the `impl Default for Theme` block
+* all customizable theme elements can be found in [`style.rs` in the `impl Default for Theme` block](https://github.com/extrawurst/gitui/blob/master/src/ui/style.rs#L305)