Revert the change of InactiveBorderColor to "dim"

This was an attempt to make lazygit's overall UI look more "elegant",
but it has too many problems. The biggest is that we have panels that
combine two views (e.g. a prompt with suggestions below, or the commit
message editor with subject and body), and showing the frame of the
inactive half of these as "dim" looks wrong; we'd have to add a new
concept of a view that doesn't have the focus but otherwise still looks
active.

Also, some terminals don't do a good job of showing lines made of dim
box drawing characters (e.g. Zed's builtin terminal). This could
probably be improved by using a grey hex color instead of dim, but then
we'd have to pick one that looks good with all possible terminal color
scheme.

That's too many questions, and it's not important enough to spend time
on this, so just revert the change.

docs-master/Config.md

@@ -181,7 +181,7 @@ gui:
 
     # Border color of non-focused windows
     inactiveBorderColor:
-      - dim
+      - default
 
     # Border color of focused window when searching in that window
     searchingActiveBorderColor:

pkg/config/user_config.go

@@ -897,7 +897,7 @@ func GetDefaultConfigForPlatform(platform string) *UserConfig {
 			Theme: ThemeConfig{
 				ActiveBorderColor:          []string{"green", "bold"},
 				SearchingActiveBorderColor: []string{"cyan", "bold"},
-				InactiveBorderColor:        []string{"dim"},
+				InactiveBorderColor:        []string{"default"},
 				OptionsTextColor:           []string{"blue"},
 				SelectedLineFgColor:        []string{"bold"},
 				CherryPickedCommitBgColor:  []string{"cyan"},

schema-master/config.json

@@ -739,7 +739,7 @@
               "uniqueItems": true,
               "description": "Border color of non-focused windows",
               "default": [
-                "dim"
+                "default"
               ]
             },
             "searchingActiveBorderColor": {