Enhance looks and feel of Ui

[chonguschonguschongus]

Change colour scheme of Ui to fit a more hospital kind of vibes.

Add in a sneaky User Guide update as well.

[codecov]

Codecov Report

Merging #152 (e394926) into master (8c330cc) will decrease coverage by 0.22%.
The diff coverage is 0.00%.

❗ Current head e394926 differs from pull request most recent head 1688882. Consider uploading reports for the commit 1688882 to get more accurate results

@@             Coverage Diff              @@
##             master     #152      +/-   ##
============================================
- Coverage     66.37%   66.16%   -0.22%     
  Complexity      638      638              
============================================
  Files           111      111              
  Lines          2513     2521       +8     
  Branches        253      253              
============================================
  Hits           1668     1668              
- Misses          748      756       +8     
  Partials         97       97              

Files	Coverage Δ
src/main/java/seedu/address/ui/DoctorCard.java	0.00% <0.00%> (ø)
src/main/java/seedu/address/ui/PatientCard.java	0.00% <0.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more