theme: Use a matching layout theme for any widget theme

Dedicated styling is now possible for themes which have a corresponding style-theme.css file. Adwaita:dark gets one, whereas other themes use the new generic fallback theme.

data/style-Adwaita:dark.css

@@ -0,0 +1,46 @@
+sq_view {
+    background-color: rgba(0, 0, 0, 255);
+    color: #ffffff;
+    font-family: cantarell, sans-serif;
+}
+
+sq_view sq_button {
+    color: #deddda;
+    background: #464448;
+    border-style: solid;
+    border-width: 1px;
+    border-color: #5e5c64;
+    border-radius: 3px;
+    margin: 4px 2px 4px 2px;
+}
+
+sq_view.wide sq_button {
+    margin: 1px 1px 1px 1px;
+}
+
+sq_button:active {
+    background: #747077;
+    border-color: #96949d;
+}
+
+sq_button.altline,
+sq_button.special,
+sq_button.wide {
+    background: #2b292f;
+    border-color: #3e3a44;
+}
+
+sq_button.locked {
+    background: #ffffff;
+    color: #2b292f;
+}
+
+#Return {
+    background: #1c71d8;
+    border-color: #1a5fb4;
+}
+
+#Return:active {
+    background: #1c71d8;
+    border-color: #3584e4;
+}