Use accent colour to distinguish last read indicator.

ts/components/conversation/SessionLastSeenIndicator.tsx

@@ -26,7 +26,7 @@ const LastSeenText = styled.div`
   text-transform: uppercase;
   text-align: center;
 
-  color: var(--color-last-seen-indicator-text);
+  color: #00f782;
 `;
 
 export const SessionLastSeenIndicator = (props: { messageId: string }) => {

ts/state/ducks/SessionTheme.tsx

@@ -28,7 +28,7 @@ const darkColorSessionBorder = `1px solid ${borderDarkThemeColor}`;
 const darkColorSessionBorderColor = borderDarkThemeColor;
 const darkColorRecoveryPhraseBannerBg = '#1f1f1f';
 const darkColorPillDivider = '#353535';
-const darkColorLastSeenIndicator = '#353535';
+const darkColorLastSeenIndicator = accentDarkTheme;
 const darkColorLastSeenIndicatorText = '#a8a9aa';
 const darkColorQuoteBottomBarBg = '#404040';
 const darkColorCellBackground = '#1b1b1b';