Rename Default theme to Light theme

[Ma-Yueran]

The name of the previous Default theme is changed to be Light theme, so making Dark Theme the default theme is no longer an error.

[codecov-io]

Codecov Report

Merging #271 into master will not change coverage.
The diff coverage is 66.66%.

@@            Coverage Diff            @@
##             master     #271   +/-   ##
=========================================
  Coverage     64.42%   64.42%           
  Complexity      550      550           
=========================================
  Files           100      100           
  Lines          1816     1816           
  Branches        189      189           
=========================================
  Hits           1170     1170           
  Misses          572      572           
  Partials         74       74           

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/seedu/address/ui/ThemeWindow.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
src/main/java/seedu/address/ui/theme/ThemeSet.java	83.33% <100.00%> (ø)	3.00 <1.00> (ø)

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[luo-git]

LGTM!