Match dark/light mode of Joplin

[shbach]

Add ability for plugin to match light and dark modes of Joplin which will look better and more cohesive.

[agerardin]

With v1.0.3. the control panel should be styled according to the Theme. That would be a bit trickier for the graph itself since Joplin exposes CSS variables for styling but the graph is rendered in a canvas element (canvas draws on pixels so CSS is not an option out of the box). Also this would mean figuring out color schemes that are pleasing for each theme and each extension moving forward. Doable but also more things to test for at each release.

[shbach]

Nice, it's looking more cohesive already.

[shbach]

One more thing that can be worked on is for the newly added collapsible menu, it can be made to more closely match the scrollbars used by Joplin. In dark mode, the light color of the scrollbar stands out.