Updated stylesheet with mobile & dark theme

[GeoDerp]

Light and Dark Theme

Colour scheme has been simplified (sticking to the silver and dark blue theme), icons added & buttons rounded to generate a "modern" design. Adaptive Dark mode style has been added to the .css to support dark themed devices. (currently only tested on Firefox)

Light

Dark

Mobile

Scrolling overflow has been added to support for easier navigation on small screened devices

Screenshot

Known ToDo (optional)

• [ ] mod menu buttons are positioned differently according to screen aspect ratio. This may result in the menu being positioned in odd/overlapping places on certain screens. Hopefully a visual issue only.
• [ ] make a Dark theme for Diagrams

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5691360) 89.52% compared to head (c4706b4) 89.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #146 +/- ## ======================================= Coverage 89.52% 89.52% ======================================= Files 6 6 Lines 1442 1442 ======================================= Hits 1291 1291 Misses 151 151 ```

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[davidusb-geek]

Thanks for these improvements...