libsysprof-gtk: make accent fg/bg color available internally

This does more work than it should have to since we are reading the color
repeatedly, but it's still better than the data being stored per-instance.

src/libsysprof-gtk/sysprof-chart-layer-private.h

@@ -0,0 +1,30 @@
+/* sysprof-chart-layer-private.h
+ *
+ * Copyright 2023 Christian Hergert <chergert@redhat.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#include "sysprof-chart-layer.h"
+
+G_BEGIN_DECLS
+
+const GdkRGBA *_sysprof_chart_layer_get_accent_bg_color (void);
+const GdkRGBA *_sysprof_chart_layer_get_accent_fg_color (void);
+
+G_END_DECLS