Invert webview colors when using dark/black theme

FAQ.md

@@ -147,7 +147,7 @@ FairEmail follows all the best practices for an email client as decribed in [thi
 * [(78) How do I use schedules?](#user-content-faq78)
 * [(79) How do I use synchronize on demand (manual)?](#user-content-faq79)
 * [(80) How can I fix 'Unable to load BODYSTRUCTURE'?](#user-content-faq80)
-* [(81) Can you make the background of the original message dark in the dark theme?](#user-content-faq81)
+* [~~(81) Can you make the background of the original message dark in the dark theme?~~](#user-content-faq81)
 * [(82) What is a tracking image?](#user-content-faq82)
 * [(83) What does 'User is authenticated but not connected' mean?](#user-content-faq83)
 * [(84) What are local contacts for?](#user-content-faq84)
@@ -1375,10 +1375,12 @@ FairEmail already tries to workaround these bugs, but if this fail you'll need t
 <br />
 
 <a name="faq81"></a>
-**(81) Can you make the background of the original message dark in the dark theme?**
+**~~(81) Can you make the background of the original message dark in the dark theme?~~**
 
-The original message is shown as the sender has sent it, including all colors.
-Changing the background color would not only make the original view not original anymore, it can also result in unreadable messages.
+~~The original message is shown as the sender has sent it, including all colors.~~
+~~Changing the background color would not only make the original view not original anymore, it can also result in unreadable messages.~~
+
+Recent versions of FairEmail will invert all colors of the original message when using a dark or black theme.
 
 <br />
 

app/src/main/java/eu/faircode/email/AdapterMessage.java

@@ -36,6 +36,8 @@ import android.content.pm.PackageManager;
 import android.content.res.ColorStateList;
 import android.database.Cursor;
 import android.graphics.Color;
+import android.graphics.ColorMatrixColorFilter;
+import android.graphics.Paint;
 import android.graphics.Rect;
 import android.graphics.Typeface;
 import android.graphics.drawable.Drawable;
@@ -139,6 +141,7 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
     private boolean suitable;
     private IProperties properties;
 
+    private boolean dark;
     private boolean date;
     private boolean threading;
     private boolean contacts;
@@ -1398,6 +1401,19 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
                     }
                 };
 
+                if (dark) {
+                    float[] NEGATIVE = new float[]{
+                            -1, 0, 0, 0, 255, // red
+                            0, -1, 0, 0, 255, // green
+                            0, 0, -1, 0, 255, // blue
+                            0, 0, 0, 1, 0 // alpha
+                    };
+
+                    Paint paint = new Paint();
+                    paint.setColorFilter(new ColorMatrixColorFilter(NEGATIVE));
+                    webView.setLayerType(View.LAYER_TYPE_HARDWARE, paint);
+                }
+
                 webView.setWebViewClient(new WebViewClient() {
                     public boolean shouldOverrideUrlLoading(WebView view, String url) {
                         Log.i("Open url=" + url);
@@ -3033,6 +3049,9 @@ public class AdapterMessage extends RecyclerView.Adapter<AdapterMessage.ViewHold
         this.suitable = Helper.getNetworkState(context).isSuitable();
         this.properties = properties;
 
+        TypedValue tv = new TypedValue();
+        context.getTheme().resolveAttribute(R.attr.themeName, tv, true);
+        this.dark = !"light".equals(tv.string);
 
         this.date = prefs.getBoolean("date", true);
         this.threading = prefs.getBoolean("threading", true);