improve back button hover color

The back button on conversation panel has now a lighter color on
hovering. The previous one was too dark and too close to the arrow's
color.

Change-Id: I63402128ddb3fad699a4e0d552e9bf2ff92c0f4e
Reviewed-by: Olivier Soldano <olivier.soldano@savoirfairelinux.com>

ui/Base.lproj/Conversation.xib

@@ -179,11 +179,8 @@
                         <font key="font" metaFont="system"/>
                     </buttonCell>
                     <userDefinedRuntimeAttributes>
-                        <userDefinedRuntimeAttribute type="color" keyPath="highlightColor">
-                            <color key="value" red="0.16862745100000001" green="0.70588235290000001" blue="0.78823529410000004" alpha="1" colorSpace="calibratedRGB"/>
-                        </userDefinedRuntimeAttribute>
                         <userDefinedRuntimeAttribute type="color" keyPath="hoverColor">
-                            <color key="value" red="0.16078431369999999" green="0.16078431369999999" blue="0.16078431369999999" alpha="0.80000000000000004" colorSpace="calibratedRGB"/>
+                            <color key="value" red="0.76470588235294112" green="0.76470588235294112" blue="0.76470588235294112" alpha="1" colorSpace="calibratedRGB"/>
                         </userDefinedRuntimeAttribute>
                         <userDefinedRuntimeAttribute type="color" keyPath="imageColor">
                             <color key="value" white="0.0" alpha="1" colorSpace="calibratedWhite"/>