Add test cases for

[Carlintyj]

ThemeCommand, ThemeCommandParser, CourseUtil and Course

[codecov]

Codecov Report

Merging #253 (0942d20) into master (9e02455) will increase coverage by 0.24%.
The diff coverage is 83.33%.

@@             Coverage Diff              @@
##             master     #253      +/-   ##
============================================
+ Coverage     76.27%   76.52%   +0.24%     
- Complexity      668      675       +7     
============================================
  Files           101      101              
  Lines          2082     2087       +5     
  Branches        227      229       +2     
============================================
+ Hits           1588     1597       +9     
+ Misses          446      441       -5     
- Partials         48       49       +1     

Files	Coverage Δ
...seedu/address/logic/parser/ThemeCommandParser.java	100.00% <100.00%> (+18.75%)	⬆️
...ava/seedu/address/logic/commands/ThemeCommand.java	83.33% <85.71%> (-16.67%)	⬇️
...main/java/seedu/address/model/util/CourseUtil.java	83.33% <0.00%> (-7.58%)	⬇️

... and 1 file with indirect coverage changes

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[leontan2]

LGTM!