Fix plugin visuals to handle different themes

[colorfusion]

Summary

This pull request will adjust the visuals of the plugin, allowing the plugin to adapt to different themes in the application accordingly.
The style has been adjusted slightly to differentiate the different components in the plugin, and to follow Mattermost's layout more closely.

Ticket Link

Fixes #78

Screenshot

Sidebar

Add / Request Todo (root page)

Todo Message Input

[codecov-commenter]

Codecov Report

Merging #116 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #116   +/-   ##
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  Coverage    3.97%   3.97%           
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  Files           9       9           
  Lines         930     930           
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  Hits           37      37           
  Misses        893     893           

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[larkox]

Awesome job, @colorfusion ! Thank you very much.

LGMT

[asaadmahmood]

@colorfusion We only use theme variables in the multiples of 4.

[asaadmahmood]

@colorfusion We only use theme variables in the multiples of 4.

You can see the variables available in the utils file of the webapp:
https://github.com/mattermost/mattermost-webapp/blob/master/utils/utils.jsx

[colorfusion]

I've adjusted the theme variables to be in multiple of 4, and fixed the input box's border with new screenshots in the PR.
Do let me know if there are any additional changes that should be made as well.

[hanzei]

LGTM 👍

[larkox]

LGTM!

[DHaussermann]

Tested and passed

• Tabs now look like tabs and not buttons
• Typeface color on unselected tab now has more contrast on darker themes to be more readable
• Tested all themes
• Color scheme of To Do seems consistent with other UI elements when theme is applied
• No issues found
  LGTM!

Huge thanks @colorfusion for implementing this! Your changes have hugely improved the UI for this plugin. Much appreciated.