Refactor screen rotation setup and GTK theme configuration in first-boot process

Update the one-shot script to set screen rotation using kanshi based on kernel command line parameters, replacing the previous wlr-randr method. The script now writes the configuration to ~/.config/kanshi/config and sets the GTK dark theme (PiXnoir or Adwaita-dark) at first login. Additionally, enhance documentation to reflect these changes and clarify the role of the new script in the first-boot process.

emmc-provisioning/cloud-init/config-files/panel-theme.css

@@ -0,0 +1,44 @@
+/* Custom wf-panel-pi theme – dark, rounded panel
+ * Place at ~/.config/wf-panel-pi/panel-theme.css
+ * Set css_path in wf-panel-pi.ini to this file (absolute path).
+ */
+
+/* Panel window */
+window {
+  background-color: transparent;
+}
+
+/* Main panel box – dark with slight rounding and shadow */
+window box {
+  background-color: rgba(45, 45, 45, 0.96);
+  border-radius: 10px;
+  margin: 4px;
+  padding: 2px 8px;
+  border: 1px solid rgba(255, 255, 255, 0.06);
+  box-shadow: 0 2px 10px rgba(0, 0, 0, 0.35);
+}
+
+/* Plugin / launcher buttons */
+button,
+button:hover,
+button:active {
+  background: transparent;
+  border: none;
+  border-radius: 6px;
+  padding: 4px;
+  color: #e0e0e0;
+}
+
+button:hover {
+  background: rgba(255, 255, 255, 0.08);
+}
+
+button:active {
+  background: rgba(255, 255, 255, 0.12);
+}
+
+/* Labels (e.g. clock text) */
+label {
+  color: #e0e0e0;
+  font-size: 11pt;
+}