Refactor Material Theme

[sc1f]

• split out common colors & fonts into variables.less
• restructure and refactor material.less & material.dark.less for readability and reduction of duplicate rule declaration.

[timkpaine]

make sure jlab picks these up https://github.com/jpmorganchase/perspective/tree/master/packages/perspective-jupyterlab/src/css

[texodus]

Small tweak

[texodus]

These seem to revert a commit I made on responsive-sidebar before merging, to consolidate .columns_horizontal.

[texodus]

Nevermind - I misread, this works as intended.

[timkpaine]

I didnt know you could do this...

[sc1f]

Changes fixed.

[texodus]

Might be worth putting style rules for perspective-computed-column in a future commit rather than here.

[sc1f]

@timkpaine the Jlab css looks exactly like the compiled version of the .less files in perspective-viewer. The compilation of less > css allows me to do the refactoring because I can reduce bulk through the less syntax, but I can't necessarily do that in straight CSS. We could try to set up .less compilation for Jlab but that seems to be outside the scope of this PR.

[sc1f]

@texodus looks like the travis/push build broke on the flaky perspective-hypergrid test. Is there a way to manually run that check again without pushing to the repo/updating the PR?

[timkpaine]

@sc1f I reran the build, passed now.

[texodus]

Looks good, thanks!