stylesheet: Fix accent colour on levelbars and calendar widgets. Closes #1134

This MR adds the accent colours on levelbars and calendar widgets, as observed in #1134 (closed).

This also gets rid of the black border on the levelbars.

One little difference is in the border colour of the levelbar: in Adwaita, the colour is a slightly darker shade of blue, while in this commit is the same colour as the accent. If needed, we can amend this to darken the accent colour on the border (but I personally prefer having the same colour). (not true 🤦, both the border and the background colour needs to be changed though)