added gtk3 theming

.config/gtk-3.0/bookmarks

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+file:///mnt/network/Video
+file:///mnt/backup
+file:///home/marc/Archiv/Downloads

.config/gtk-3.0/gtk.css

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+/*
+ * see here for theming tutorial
+ * https://ubuntu-mate.community/t/enabling-the-gtk-inspector-to-find-css-values-for-theme-styles/20150
+ * for an example here
+ * https://github.com/xi/gtk-theme-clearcodes/tree/master/gtk-3.0/widgets
+*/
+
+@define-color bg_color #21252b;
+@define-color fg_color #a9b2c3;
+@define-color acc1_color #b9ca4a;
+@define-color acc2_color #61afef;
+
+@import url("shared.css");
+
+@import url("widgets/menu.css");

.config/gtk-3.0/shared.css

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+/*
+ * see here for theming tutorial
+ * https://ubuntu-mate.community/t/enabling-the-gtk-inspector-to-find-css-values-for-theme-styles/20150
+*/
+
+* {
+    border-color: @bg_color;
+    border-style: solid;
+
+    -GtkTreeView-expander-size: 11;
+}
+
+selection,
+*:selected:selected,
+*:selected:focus {
+    background-color: @acc1_color;
+    color: @bg_color;
+}
+
+*:backdrop:backdrop,
+*:disabled:disabled {
+    color: mix(@fg_color, @bg_color, 0.4);
+    -gtk-icon-effect: dim;
+}
+
+.background {
+    background: @bg_color;
+}
+
+.view,
+.cell {
+    background: @bg_color;
+    color: @fg_color;
+}
+
+entry {
+    padding: 3px;
+    border-width: 1px;
+    background-color: @bg_color;
+    color: @fg_color;
+}
+
+entry:backdrop,
+entry:disabled {
+    background: shade(@bg_color, 0.94);
+}
+
+slider:focus,
+switch:focus slider,
+spinbutton:focus,
+notebook:focus tab:checked,
+radio:focus,
+check:focus,
+entry:focus,
+button:focus {
+    border-color: @acc2_color;
+}
+
+/*
+.thunar toolbar {
+    color: @fg_color;
+    background-color: @bg_color;
+}
+
+.thunar .sidebar .view,
+.thunar .sidebar .iconview {
+    color: @fg_color;
+    background-color: @bg_color;
+}
+
+.thunar .sidebar .view:selected,
+.thunar .sidebar .iconview:selected,
+.thunar .sidebar .view:active,
+.thunar .sidebar .iconview:active {
+    color: @bg_color;
+    background-color: @acc1_color;
+}
+
+.thunar .standard-view .view {
+    color: @fg_color;
+    background-color: @bg_color;
+}
+
+.thunar .standard-view .view:selected,
+.thunar .standard-view .iconview:selected,
+.thunar .standard-view .view:active,
+.thunar .standard-view .iconview:active {
+    color: @bg_color;
+    background-color: @acc1_color;
+}
+
+@import url("widgets/menu.css");
+
+*/

.config/gtk-3.0/widgets/menu.css

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+menubar,
+actionbar,
+toolbar,
+placessidebar list {
+    background: @bg_color;
+    color: @fg_color;
+}
+
+toolbar {
+    padding: 1px;
+}
+
+actionbar {
+    padding: 0.5em;
+    border-top-width: 1px;
+}
+
+.inline-toolbar {
+    padding: 0.5em;
+    border-width: 0 1px 1px;
+}
+
+menu {
+    background: @bg_color;
+    padding: 0.4em 0.4em;
+}
+
+menu menuitem {
+    background: @bg_color;
+    color: @fg_color;
+    padding: 0.4em;
+    margin: 1px;
+}
+
+menuitem:hover {
+    background: @acc1_color;
+    color: @bg_color;
+}
+
+menuitem:hover > *,
+combobox menuitem:hover * {
+    color: @bg_color;
+}
+
+actionbar button,
+toolbar button {
+    padding: 0.2em;
+}
+
+menu separator {
+    background-color: @acc2_color;
+    margin: 0.2em;
+    min-width: 1px;
+    min-height: 1px;
+}