Various dark theme fixes (#10416)

- beatify simplemde, remove header hover effect (does not play well with
  border colors), remove double border separators
- fix colored outline buttons
- fix disabled style
- fix positive messages
- fix markdown header border and quote block
- adjust footer color
- adjust file icons and release list timeline

web_src/less/_editor.less

@@ -24,3 +24,11 @@
     border-right: 0;
     border-bottom: 0;
 }
+
+.editor-toolbar {
+    opacity: 1 !important;
+}
+
+.editor-toolbar i.separator {
+    border-left: none;
+}