🎨 Add theme support

- Move root (var) style properties to `/assets/css/themes.scss`
- Improve mobile navigation
- Create section component for the collapsable sections.
- Create logo component, so color can be changed.

- Add settings page
- Add option to select theme color
- Add option to select theme background
- Add option to enable/disable multi-colored frames.

- Add settings to VueX Store
- Persist VueX store in LocalStorage.

store/index.js

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+export default {
+
+}

store/postwoman.js

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+export const SETTINGS_KEYS = [
+    /**
+     * The CSS class that should be applied to the root element.
+     * Essentially, the name of the background theme.
+     */
+    "THEME_CLASS",
+
+    /**
+     * The hex color code for the currently active theme.
+     */
+    "THEME_COLOR",
+
+    /**
+     * Whether or not THEME_COLOR is considered 'vibrant'.
+     *
+     * For readability reasons, if the THEME_COLOR is vibrant,
+     * any text placed on the theme color will have its color
+     * inverted from white to black.
+     */
+    "THEME_COLOR_VIBRANT",
+
+    /**
+     * Normally, section frames are multicolored in the UI
+     * to emphasise the different sections.
+     * This setting allows that to be turned off.
+     */
+    "DISABLE_FRAME_COLORS"
+];
+
+export const state = () => ({
+    settings: {}
+});
+
+export const mutations = {
+
+    applySetting (state, setting) {
+        if(setting == null || !(setting instanceof Array) || setting.length !== 2)
+            throw new Error("You must provide a setting (array in the form [key, value])");
+
+        let key = setting[0];
+        let value = setting[1];
+        // Do not just remove this check.
+        // Add your settings key to the SETTINGS_KEYS array at the
+        // top of the file.
+        // This is to ensure that application settings remain documented.
+        if(!SETTINGS_KEYS.includes(key)) throw new Error("The settings key does not include the key " + key);
+
+        state.settings[key] = value;
+    }
+
+};