Improve border parsing in themes

The `top`, `left`, `bottom` and `right` fields are optional.
If none are specified, then they are all set to true.

Co-Authored-By: Nathan Sobo <nathan@zed.dev>

zed/assets/themes/_base.toml

@@ -4,7 +4,7 @@ background = "$elevation_1"
 [ui.tab]
 background = "$elevation_2"
 text = "$text_dull"
-border.color = "#000000"
+border = { color = "#000000", width = 1.0 }
 padding = { left = 10, right = 10 }
 icon_close = "#383839"
 icon_dirty = "#556de8"

zed/assets/themes/dark.toml

@@ -10,7 +10,7 @@ text_bright = "#ffffff"
 text_normal = "#d4d4d4"
 
 [syntax]
-keyword = { color = "#c586c0", weight = "bold" }
+keyword = { color = "#0086c0", weight = "bold" }
 function = "#dcdcaa"
 string = "#cb8f77"
 type = "#4ec9b0"