fix: remove body background-color/color transitions to prevent theme flash

- Remove transition: background-color/color from body in all CSS files
- These transitions caused visible flash when navigating between pages
- The browser would animate from old theme colors to new theme colors

pos/static/css/customers.css

@@ -20,8 +20,6 @@
       font-weight: var(--font-weight-regular);
       background-color: var(--color-bg-base);
       color: var(--color-text-primary);
-      transition: background-color var(--duration-normal) var(--ease-in-out),
-                  color var(--duration-normal) var(--ease-in-out);
       display: flex;
       flex-direction: column;
       overflow: hidden;