fix(trace): dark mode arrow (#67593)

Arrow was invisible in dark mode :)

static/app/views/performance/newTraceDetails/trace.tsx

@@ -2191,6 +2191,10 @@ const TraceStylingWrapper = styled('div')`
     display: flex;
     align-items: center;
 
+    svg {
+      fill: ${p => p.theme.subText};
+    }
+
     &.Left {
       left: 0;
     }