fix: make slash command labels readable in light theme (#29512)

The `/` command menu is portalled into a tippy popup by
getSuggestionRenderer, and tippy's base stylesheet sets
.tippy-box { color: #fff }. The app's `transparent` theme neutralises
tippy's background, padding and margin but never its text colour, so
everything inside the popup inherits white.

DropdownMenu paints bg-white dark:bg-gray-850 dark:text-white, defining
a text colour only for dark mode, and the command rows in SlashCommands
set no colour of their own. In light theme that leaves Temporary, Model
and Settings as white text on a white menu.

Reset the colour on the transparent theme so the popup inherits from
body, which is #000 in light and #eee in dark. The secondary text and
the prompt rows were already unaffected because they set explicit
theme-aware colours.

src/app.css

@@ -715,7 +715,7 @@ html.high-contrast.dark .ProseMirror p.is-editor-empty:first-child::before {
 }
 
 .tippy-box[data-theme~='transparent'] {
-	@apply bg-transparent p-0 m-0;
+	@apply bg-transparent p-0 m-0 text-inherit;
 }
 
 /* this is a rough fix for the first cursor position when the first paragraph is empty */