Very minor UI color suggestion for the monochrome theme

[Sunspark-007]

Steps to reproduce

@MajorTanya In the upper right corner of the library the filter/sort/display icon is yellow if an item is checked, as opposed to being grey.

Expected behavior

Yellow icon should be grey.

Actual behavior

Icon is yellow if an item is checked.

Crash logs

No response

Mihon version

0.18.0

Android version

Android 9

Device

Samsung A70

Other details

Relates to #1752

Acknowledgements

• I have searched the existing issues and this is a new ticket, NOT a duplicate or related to another open or closed issue.
• I have written a short but informative title.
• I have gone through the FAQ and troubleshooting guide.
• I have updated the app to version 0.18.0.
• I have filled out all of the requested information in this form, including specific version numbers.
• I understand that Mihon does not have or fix any extensions, and I will not receive help for any issues related to sources or extensions.