Fix color of the “No description added“ media upload warning on light theme (#20328)

app/javascript/styles/mastodon-light/diff.scss

@@ -203,7 +203,8 @@ html {
 // Change the colors used in compose-form
 .compose-form {
   .compose-form__modifiers {
-    .compose-form__upload__actions .icon-button {
+    .compose-form__upload__actions .icon-button,
+    .compose-form__upload__warning .icon-button {
       color: lighten($white, 7%);
 
       &:active,
@@ -212,14 +213,6 @@ html {
         color: $white;
       }
     }
-
-    .compose-form__upload-description input {
-      color: lighten($white, 7%);
-
-      &::placeholder {
-        color: lighten($white, 7%);
-      }
-    }
   }
 
   .compose-form__buttons-wrapper {