Reorganize Chroma styles (#13934)

This moves the Chroma styles into separate files with base,light and
dark variants. Should work exactly as before.

web_src/less/chroma/base.less

@@ -0,0 +1,50 @@
+.chroma {
+  background-color: var(--color-code-bg);
+}
+
+/* LineTableTD */
+.chroma .lntd {
+  vertical-align: top;
+  padding: 0;
+  margin: 0;
+  border: 0;
+}
+
+/* LineTable */
+.chroma .lntable {
+  border-spacing: 0;
+  padding: 0;
+  margin: 0;
+  border: 0;
+  width: auto;
+  overflow: auto;
+  display: block;
+}
+
+/* LineHighlight */
+.chroma .hl {
+  display: block;
+  width: 100%;
+}
+
+/* LineNumbersTable */
+.chroma .lnt {
+  margin-right: .4em;
+  padding: 0 .4em;
+}
+
+/* LineNumbers */
+.chroma .ln {
+  margin-right: .4em;
+  padding: 0 .4em;
+}
+
+/* GenericStrong */
+.chroma .gs {
+  font-weight: 600;
+}
+
+/* GenericUnderline */
+.chroma .gl {
+  text-decoration: underline;
+}