Add warning message when using old thread design

In threads which are using nested replies, a warning will be shown if
the user has enabled the old thread design to notify them that they
won't work properly.

Fixed: twpowertools:139
Change-Id: I393ac187fef22a4c5f062bf99d84c373011d514b
diff --git a/src/static/css/ccdarktheme.css b/src/static/css/ccdarktheme.css
index be6b4d9..ddc4b3f 100644
--- a/src/static/css/ccdarktheme.css
+++ b/src/static/css/ccdarktheme.css
@@ -2289,3 +2289,7 @@
 .TWPT-log-entry.TWPT-log-entry--error {
   color: #ff8A80!important;
 }
+
+.TWPT-warning {
+  background-color: #670505!important;
+}
diff --git a/src/static/css/thread_page_design_warning.css b/src/static/css/thread_page_design_warning.css
new file mode 100644
index 0000000..71e4989
--- /dev/null
+++ b/src/static/css/thread_page_design_warning.css
@@ -0,0 +1,32 @@
+.TWPT-warning {
+  display: flex;
+  flex-direction: row;
+  align-items: center;
+
+  padding: 8px 48px;
+  margin-bottom: 16px;
+  border-radius: 8px;
+
+  background-color: #feecec;
+}
+
+.TWPT-warning--icon {
+  margin-right: 8px;
+}
+
+.TWPT-warning--text {
+  flex-grow: 1;
+}
+
+.TWPT-warning--btn {
+  display: flex;
+  flex-direction: row;
+  align-items: center;
+  min-width: 100px;
+  margin-left: 8px;
+}
+
+.TWPT-warning--btn .TWPT-badge {
+  --icon-size: 14px;
+  margin-right: 4px;
+}