Diff syntax coloring per theme: additions, removals, muted context.
DEFAULT_CSS = '\n DiffMessage {\n height: auto;\n padding: 1;\n margin: 0 0 1 0;\n background: $surface;\n border: solid $primary;\n }\n\n DiffMessage .diff-header {\n text-style: bold;\n margin-bottom: 1;\n }\n\n DiffMessage .diff-add {\n color: $text-success;\n background: $success-muted;\n }\n\n DiffMessage .diff-remove {\n color: $text-error;\n background: $error-muted;\n }\n\n DiffMessage .diff-context {\n color: $text-muted;\n }\n\n DiffMessage .diff-hunk {\n color: $secondary;\n text-style: bold;\n }\n '