Fixed a bug that prevented the addon from working with Lemmy 0.19.5

Fixes for Lemmy 0.19.3 dark theme:
- fixed incorrect text color in tables
- fixed colors of new code blocks

Improvements:
- automatic instance detection - all Lemmy instances are now supported

Improvements:
- added missing backgrounds for inbox, login and signup page

Fixes for Lemmy 0.18.2 dark theme:
- fixed incorrect button colors
- fixed some of the backgrounds using incorrect colors in some cases

Improvements:
- turned the sidebar "block" button to outline button in 0.18

Fixes for Lemmy 0.18.1-rc.7:
- added missing background for comments and communities
- fixed reply form buttons not being outline buttons

Style improvements:
- added a background for 0.18 modlog and instance list
- improved colors in dark theme
- removed top border in comments
- removed styles from card header to make it less distracting

Style improvements for Lemmy 0.17 and 0.18.