Qt/Send: Figure a decent warning colour from theme

src/qt/forms/sendcoinsdialog.ui

@@ -764,9 +764,11 @@
             <property name="toolTip">
              <string>Using the fallbackfee can result in sending a transaction that will take serval hours or days (or never) to confirm. Consider choosing your fee manually or wait until your have validated the complete chain.</string>
             </property>
-            <property name="styleSheet">
+            <property name="font">
-             <string notr="true">color: rgb(255, 150, 0);
+            <font>
-font-weight: bold;</string>
+                <weight>75</weight>
+                <bold>true</bold>
+            </font>
             </property>
             <property name="text">
              <string>Warning: Fee estimation is currently not possible.</string>

src/qt/sendcoinsdialog.cpp

@@ -656,6 +656,9 @@ void SendCoinsDialog::updateSmartFeeLabel()
         ui->labelSmartFee2->show(); // (Smart fee not initialized yet. This usually takes a few blocks...)
         ui->labelFeeEstimation->setText("");
         ui->fallbackFeeWarningLabel->setVisible(true);
+        int lightness = ui->fallbackFeeWarningLabel->palette().color(QPalette::WindowText).lightness();
+        QColor warning_colour(255 - (lightness / 5), 176 - (lightness / 3), 48 - (lightness / 14));
+        ui->fallbackFeeWarningLabel->setStyleSheet("QLabel { color: " + warning_colour.name() + "; }");
     }
     else
     {