Improve color support for themes

[VictorLamoine]

This improves readability when using dark or other themes in RViz.

Before

White theme

Dark theme

There are some other colors that might be improved but this fixes the main issues I have seen.

---

Does someone know where the color is set for the status tree and similar entries?

[VictorLamoine]

Firendly 🐷

[rhaschke]

Heading into the right direction. To fix the remaining colors, search for QColor in the source tree 😉

[VictorLamoine]

I hard a hard time finding this color because it was not initialized.
This pull request is ready on my side.

After

White theme

Black theme

[VictorLamoine]

In the help panel:

This panel uses HTML so it's not easy to fix. We can force the color of the text in the tabs like this: (I have only done the first one on the screenshot below)

  <tr class="odd">
    <td class="key">M</td>
    <td><font color="black">Switch to the Move-Camera tool.</font></td>
  </tr>
  <tr class="even">

It's not very elegant but it works.

[VictorLamoine]

@rhaschke can you have an other look?

[rhaschke]

I will have another look, but probably not earlier than March 9th.

[rhaschke]

Dear @VictorLamoine,
I further tweaked the coloring and cleaned up the commit history in my own fork:
https://github.com/rhaschke/rviz/tree/colors_dark_theme

Could you please git-reset your branch to this one and verify that everything looks fine for you as well?
As you see from the images below, the panel title bars are not correctly colored on my side - probably due to a bug in the Adwaita-dark theme that I'm using for testing. On your side it looked good.

bright:

dark:

[VictorLamoine]

Most likely you adwaita theme is incomplete!

RViz with Kubuntu breeze theme look nice:

[PeterMitrano]

how do I use this?

[rhaschke]

Just change your global desktop theme to a dark one.

[PeterMitrano]

Hm.. I think I've done that. For instance, nautilus and jstest-gtk is in dark theme. But rviz is not (even after restarting). I am on ROS Melodic on Ubuntu 18.04
rviz version 1.13.12

[rhaschke]

This might have run into a well-known Qt issue: In order to use gtk desktop themes, you need to install qt5-style-plugins and maybe
export QT_QPA_PLATFORMTHEME=gtk2

[PeterMitrano]

that did the trick -- and setting that variable seems nessecary. Thanks!