UI fixes

[TheStaticBits]

• route selector gradient
• switch settings menu theme selector UI for consistency
• dark theme for slide selector
• hide center location button when location is centered
• fix search options blocking whole page

[TheStaticBits]

my current setup for hiding the center location button when the user location is visible on screen could be slow depending on how fast it is for flutter to get the user's location. It can also be slightly glitchy from my testing. It checks against the user's location and the new viewport when the user moves the camera