Fix: Switch the default theme from light mode to dark mode and improve some styles #9869 (#10552)

### What problem does this PR solve?

Fix: Switch the default theme from light mode to dark mode and improve
some styles #9869
-Update UI component styles such as input boxes, tables, and prompt
boxes
-Optimize login page layout and style details
-Revise some of the wording, such as uniformly changing "data flow" to
"pipeline"
-Adjust the parser to support the markdown type

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

web/src/app.tsx

@@ -104,7 +104,7 @@ const RootProvider = ({ children }: React.PropsWithChildren) => {
     <TooltipProvider>
       <QueryClientProvider client={queryClient}>
         <ThemeProvider
-          defaultTheme={ThemeEnum.Light}
+          defaultTheme={ThemeEnum.Dark}
           storageKey="ragflow-ui-theme"
         >
           <Root>{children}</Root>