use blue color from https://mailu.io/

core/admin/Dockerfile

@@ -12,7 +12,7 @@ RUN set -eu \
 COPY webpack.config.js ./
 COPY assets ./assets
 RUN set -eu \
- && sed -i 's/#007bff/#367fa9/' node_modules/admin-lte/build/scss/_bootstrap-variables.scss \
+ && sed -i 's/#007bff/#55a5d9/' node_modules/admin-lte/build/scss/_bootstrap-variables.scss \
  && for l in ca da de:de_de en:en-gb es:es_es eu fr:fr_fr he hu is it:it_it ja nb_NO:no_nb nl:nl_nl pl pt:pt_pt ru sv:sv_se zh_CN:zh; do \
       cp node_modules/datatables.net-plugins/i18n/${l#*:}.json assets/${l%:*}.json; \
     done \

core/admin/assets/app.css

@@ -2,6 +2,9 @@
 .mailu-logo {
   opacity: .8;
 }
+.bg-mailu-logo {
+  background-color: #2980b9!important;
+}
 
 /* user image */
 .div-circle {