theme: set default loader background (#102061)

.loading.triangle styles are missing so the loader will have a flash of
default color when using system theme

Author: JonasBa

static/app/styles/global.tsx

@@ -134,26 +134,16 @@ const styles = (theme: Theme, isDark: boolean) => css`
       z-index: ${theme.zIndex.sentryErrorEmbed};
     }
 
-    color: ${theme.textColor};
-    background: ${theme.backgroundSecondary};
-  }
-
-  body.theme-dark {
-    background: ${theme.tokens.background.primary};
-
     .loading .loading-indicator {
       background: ${theme.tokens.background.primary};
     }
-  }
 
-  body.theme-system {
-    @media (prefers-color-scheme: dark) {
-      background: ${theme.tokens.background.primary};
+    .loading.triangle .loading-indicator {
+      background: ${theme.tokens.background.secondary};
     }
 
-    .loading .loading-indicator {
-      background: ${theme.tokens.background.primary};
-    }
+    color: ${theme.textColor};
+    background: ${theme.tokens.background.primary};
   }
 
   abbr {