Various style tweaks (#13418)

- Change code review '+' to SVG and increase size slightly
- Set placeholder color in both themes
- Set proper font for textareas
- Fix black code in arc-green
- Various arc-green fixes

web_src/less/_base.less

@@ -99,6 +99,8 @@ h5,
 .ui.form input[type="text"],
 .ui.form input[type="time"],
 .ui.form input[type="url"],
+.ui.input textarea,
+.ui.form textarea,
 .ui.header,
 .ui.items > .item > .content > .header,
 .ui.list .list > .item .header,
@@ -166,6 +168,12 @@ table {
   color: #fff !important;
 }
 
+::placeholder,
+.CodeMirror-placeholder {
+  color: #aaa !important;
+  opacity: 1 !important;
+}
+
 a,
 .ui.breadcrumb a {
   color: var(--color-primary);